diff --git a/.circleci/config.yml b/.circleci/config.yml
index 321431b893..b7e2021c62 100644
--- a/.circleci/config.yml
+++ b/.circleci/config.yml
@@ -762,14 +762,16 @@ jobs:
   run-test-ios-15:
     executor:
       name: macos-executor
-      xcode_version: '14.3.0'
+      # Needs to be 15.2.0 because if not running on Swift 5.9, the tests fail
+      xcode_version: '15.2.0'
 
     steps:
       - checkout
-      - set-maximum-duration
       - install-dependencies:
           install_swiftlint: false
       - update-spm-installation-commit
+      - install-runtime:
+          runtime-name: iOS 15.5
       - run:
           name: Run tests
           command: bundle exec fastlane test_ios
diff --git a/Tests/UnitTests/FoundationExtensions/LocaleExtensionsTests.swift b/Tests/UnitTests/FoundationExtensions/LocaleExtensionsTests.swift
index 526f8e0cfe..41aabae44c 100644
--- a/Tests/UnitTests/FoundationExtensions/LocaleExtensionsTests.swift
+++ b/Tests/UnitTests/FoundationExtensions/LocaleExtensionsTests.swift
@@ -40,8 +40,13 @@ class LocaleExtensionsTests: TestCase {
                 // Fixed in iOS 17 on Xcode 15
                 expect(Locale(identifier: "").rc_languageCode).to(beNil())
             } else {
-                // Broken in iOS 16 on Xcode 15
-                expect(Locale(identifier: "").rc_languageCode) == "en"
+                if #available(iOS 16.0, macOS 13.0, tvOS 16.0, watchOS 9.0, *) {
+                    // Broken in iOS 16 on Xcode 15
+                    expect(Locale(identifier: "").rc_languageCode) == "en"
+                } else {
+                    // iOS 15 on Xcode 15
+                    expect(Locale(identifier: "").rc_languageCode).to(beNil())
+                }
             }
         #else
             expect(Locale(identifier: "").rc_languageCode).to(beNil())
@@ -61,8 +66,13 @@ class LocaleExtensionsTests: TestCase {
                 // Fixed in iOS 17 on Xcode 15
                 expect(Locale(identifier: "").removingRegion).to(beNil())
             } else {
-                // Broken in iOS 16 on Xcode 15
-                expect(Locale(identifier: "").removingRegion?.identifier) == "en"
+                if #available(iOS 16.0, macOS 13.0, tvOS 16.0, watchOS 9.0, *) {
+                    // Broken in iOS 16 on Xcode 15
+                    expect(Locale(identifier: "").removingRegion?.identifier) == "en"
+                } else {
+                    // iOS 15 on Xcode 15
+                    expect(Locale(identifier: "").removingRegion).to(beNil())
+                }
             }
         #else
             expect(Locale(identifier: "").removingRegion).to(beNil())
diff --git a/Tests/UnitTests/Paywalls/PaywallDataTests.swift b/Tests/UnitTests/Paywalls/PaywallDataTests.swift
index 35e72bbd3d..ad8c4cb8d9 100644
--- a/Tests/UnitTests/Paywalls/PaywallDataTests.swift
+++ b/Tests/UnitTests/Paywalls/PaywallDataTests.swift
@@ -148,18 +148,16 @@ class PaywallDataTests: BaseHTTPResponseTest {
     }
 
     func testLocalesOrderedByPriority() throws {
-        let expected: [String]
-
-        if #available(iOS 16.0, tvOS 16.0, watchOS 9.0, *) {
-            expected = [
-                "en-US"
-            ]
-        } else {
-            expected = [
-                // `Locale.preferredLanguages` returns `en` before iOS 17.
-                "en"
-            ]
-        }
+        #if swift(>=5.9)
+        let expected = [
+            "en-US"
+        ]
+        #else
+        let expected = [
+            // `Locale.preferredLanguages` returns `en` before Xcode 15.
+            "en"
+        ]
+        #endif
 
         expect(PaywallData.localesOrderedByPriority.map(\.identifier)) == expected
     }
diff --git a/Tests/UnitTests/SubscriberAttributes/__Snapshots__/BackendSubscriberAttributesTests/iOS15-testPostReceiptWithAdServicesToken.1.json b/Tests/UnitTests/SubscriberAttributes/__Snapshots__/BackendSubscriberAttributesTests/iOS15-testPostReceiptWithAdServicesToken.1.json
index b27dfcf34a..07205fc5b1 100644
--- a/Tests/UnitTests/SubscriberAttributes/__Snapshots__/BackendSubscriberAttributesTests/iOS15-testPostReceiptWithAdServicesToken.1.json
+++ b/Tests/UnitTests/SubscriberAttributes/__Snapshots__/BackendSubscriberAttributesTests/iOS15-testPostReceiptWithAdServicesToken.1.json
@@ -14,7 +14,7 @@
     "X-Platform-Flavor" : "Unity",
     "X-Platform-Flavor-Version" : "2.3.3",
     "X-Platform-Version" : "Version 17.0.0 (Build 21A342)",
-    "X-Preferred-Locales" : "en",
+    "X-Preferred-Locales" : "en_US",
     "X-Retry-Count" : "0",
     "X-Storefront" : "USA",
     "X-StoreKit-Version" : "1",
diff --git a/Tests/UnitTests/SubscriberAttributes/__Snapshots__/BackendSubscriberAttributesTests/iOS15-testPostReceiptWithSubscriberAttributesPassesCustomerInfoIfStatusCodeIsSuccess.1.json b/Tests/UnitTests/SubscriberAttributes/__Snapshots__/BackendSubscriberAttributesTests/iOS15-testPostReceiptWithSubscriberAttributesPassesCustomerInfoIfStatusCodeIsSuccess.1.json
index 317447c27c..3a6b24730a 100644
--- a/Tests/UnitTests/SubscriberAttributes/__Snapshots__/BackendSubscriberAttributesTests/iOS15-testPostReceiptWithSubscriberAttributesPassesCustomerInfoIfStatusCodeIsSuccess.1.json
+++ b/Tests/UnitTests/SubscriberAttributes/__Snapshots__/BackendSubscriberAttributesTests/iOS15-testPostReceiptWithSubscriberAttributesPassesCustomerInfoIfStatusCodeIsSuccess.1.json
@@ -14,7 +14,7 @@
     "X-Platform-Flavor" : "Unity",
     "X-Platform-Flavor-Version" : "2.3.3",
     "X-Platform-Version" : "Version 17.0.0 (Build 21A342)",
-    "X-Preferred-Locales" : "en",
+    "X-Preferred-Locales" : "en_US",
     "X-Retry-Count" : "0",
     "X-Storefront" : "USA",
     "X-StoreKit-Version" : "1",
diff --git a/Tests/UnitTests/SubscriberAttributes/__Snapshots__/BackendSubscriberAttributesTests/iOS15-testPostReceiptWithSubscriberAttributesPassesErrorIfStatusCodeIsNotSuccess.1.json b/Tests/UnitTests/SubscriberAttributes/__Snapshots__/BackendSubscriberAttributesTests/iOS15-testPostReceiptWithSubscriberAttributesPassesErrorIfStatusCodeIsNotSuccess.1.json
index db9595da21..89eea644e3 100644
--- a/Tests/UnitTests/SubscriberAttributes/__Snapshots__/BackendSubscriberAttributesTests/iOS15-testPostReceiptWithSubscriberAttributesPassesErrorIfStatusCodeIsNotSuccess.1.json
+++ b/Tests/UnitTests/SubscriberAttributes/__Snapshots__/BackendSubscriberAttributesTests/iOS15-testPostReceiptWithSubscriberAttributesPassesErrorIfStatusCodeIsNotSuccess.1.json
@@ -14,7 +14,7 @@
     "X-Platform-Flavor" : "Unity",
     "X-Platform-Flavor-Version" : "2.3.3",
     "X-Platform-Version" : "Version 17.0.0 (Build 21A342)",
-    "X-Preferred-Locales" : "en",
+    "X-Preferred-Locales" : "en_US",
     "X-Retry-Count" : "0",
     "X-Storefront" : "USA",
     "X-StoreKit-Version" : "1",
diff --git a/Tests/UnitTests/SubscriberAttributes/__Snapshots__/BackendSubscriberAttributesTests/iOS15-testPostReceiptWithSubscriberAttributesReturnsBadJson.1.json b/Tests/UnitTests/SubscriberAttributes/__Snapshots__/BackendSubscriberAttributesTests/iOS15-testPostReceiptWithSubscriberAttributesReturnsBadJson.1.json
index 317447c27c..3a6b24730a 100644
--- a/Tests/UnitTests/SubscriberAttributes/__Snapshots__/BackendSubscriberAttributesTests/iOS15-testPostReceiptWithSubscriberAttributesReturnsBadJson.1.json
+++ b/Tests/UnitTests/SubscriberAttributes/__Snapshots__/BackendSubscriberAttributesTests/iOS15-testPostReceiptWithSubscriberAttributesReturnsBadJson.1.json
@@ -14,7 +14,7 @@
     "X-Platform-Flavor" : "Unity",
     "X-Platform-Flavor-Version" : "2.3.3",
     "X-Platform-Version" : "Version 17.0.0 (Build 21A342)",
-    "X-Preferred-Locales" : "en",
+    "X-Preferred-Locales" : "en_US",
     "X-Retry-Count" : "0",
     "X-Storefront" : "USA",
     "X-StoreKit-Version" : "1",
diff --git a/Tests/UnitTests/SubscriberAttributes/__Snapshots__/BackendSubscriberAttributesTests/iOS15-testPostReceiptWithSubscriberAttributesSendsThemCorrectly.1.json b/Tests/UnitTests/SubscriberAttributes/__Snapshots__/BackendSubscriberAttributesTests/iOS15-testPostReceiptWithSubscriberAttributesSendsThemCorrectly.1.json
index db9595da21..89eea644e3 100644
--- a/Tests/UnitTests/SubscriberAttributes/__Snapshots__/BackendSubscriberAttributesTests/iOS15-testPostReceiptWithSubscriberAttributesSendsThemCorrectly.1.json
+++ b/Tests/UnitTests/SubscriberAttributes/__Snapshots__/BackendSubscriberAttributesTests/iOS15-testPostReceiptWithSubscriberAttributesSendsThemCorrectly.1.json
@@ -14,7 +14,7 @@
     "X-Platform-Flavor" : "Unity",
     "X-Platform-Flavor-Version" : "2.3.3",
     "X-Platform-Version" : "Version 17.0.0 (Build 21A342)",
-    "X-Preferred-Locales" : "en",
+    "X-Preferred-Locales" : "en_US",
     "X-Retry-Count" : "0",
     "X-Storefront" : "USA",
     "X-StoreKit-Version" : "1",
diff --git a/Tests/UnitTests/SubscriberAttributes/__Snapshots__/BackendSubscriberAttributesTests/iOS15-testPostReceiptWithoutSubscriberAttributesSkipsThem.1.json b/Tests/UnitTests/SubscriberAttributes/__Snapshots__/BackendSubscriberAttributesTests/iOS15-testPostReceiptWithoutSubscriberAttributesSkipsThem.1.json
index b6f52b9e4b..c0c687398c 100644
--- a/Tests/UnitTests/SubscriberAttributes/__Snapshots__/BackendSubscriberAttributesTests/iOS15-testPostReceiptWithoutSubscriberAttributesSkipsThem.1.json
+++ b/Tests/UnitTests/SubscriberAttributes/__Snapshots__/BackendSubscriberAttributesTests/iOS15-testPostReceiptWithoutSubscriberAttributesSkipsThem.1.json
@@ -14,7 +14,7 @@
     "X-Platform-Flavor" : "Unity",
     "X-Platform-Flavor-Version" : "2.3.3",
     "X-Platform-Version" : "Version 17.0.0 (Build 21A342)",
-    "X-Preferred-Locales" : "en",
+    "X-Preferred-Locales" : "en_US",
     "X-Retry-Count" : "0",
     "X-Storefront" : "USA",
     "X-StoreKit-Version" : "1",
diff --git a/Tests/UnitTests/SubscriberAttributes/__Snapshots__/BackendSubscriberAttributesTests/iOS15-testPostSubscriberAttributesCallsCompletionInNetworkErrorCase.1.json b/Tests/UnitTests/SubscriberAttributes/__Snapshots__/BackendSubscriberAttributesTests/iOS15-testPostSubscriberAttributesCallsCompletionInNetworkErrorCase.1.json
index 9c20fe3176..560035efeb 100644
--- a/Tests/UnitTests/SubscriberAttributes/__Snapshots__/BackendSubscriberAttributesTests/iOS15-testPostSubscriberAttributesCallsCompletionInNetworkErrorCase.1.json
+++ b/Tests/UnitTests/SubscriberAttributes/__Snapshots__/BackendSubscriberAttributesTests/iOS15-testPostSubscriberAttributesCallsCompletionInNetworkErrorCase.1.json
@@ -14,7 +14,7 @@
     "X-Platform-Flavor" : "Unity",
     "X-Platform-Flavor-Version" : "2.3.3",
     "X-Platform-Version" : "Version 17.0.0 (Build 21A342)",
-    "X-Preferred-Locales" : "en",
+    "X-Preferred-Locales" : "en_US",
     "X-Retry-Count" : "0",
     "X-Storefront" : "USA",
     "X-StoreKit-Version" : "1",
diff --git a/Tests/UnitTests/SubscriberAttributes/__Snapshots__/BackendSubscriberAttributesTests/iOS15-testPostSubscriberAttributesCallsCompletionInSuccessCase.1.json b/Tests/UnitTests/SubscriberAttributes/__Snapshots__/BackendSubscriberAttributesTests/iOS15-testPostSubscriberAttributesCallsCompletionInSuccessCase.1.json
index 9c20fe3176..560035efeb 100644
--- a/Tests/UnitTests/SubscriberAttributes/__Snapshots__/BackendSubscriberAttributesTests/iOS15-testPostSubscriberAttributesCallsCompletionInSuccessCase.1.json
+++ b/Tests/UnitTests/SubscriberAttributes/__Snapshots__/BackendSubscriberAttributesTests/iOS15-testPostSubscriberAttributesCallsCompletionInSuccessCase.1.json
@@ -14,7 +14,7 @@
     "X-Platform-Flavor" : "Unity",
     "X-Platform-Flavor-Version" : "2.3.3",
     "X-Platform-Version" : "Version 17.0.0 (Build 21A342)",
-    "X-Preferred-Locales" : "en",
+    "X-Preferred-Locales" : "en_US",
     "X-Retry-Count" : "0",
     "X-Storefront" : "USA",
     "X-StoreKit-Version" : "1",
diff --git a/Tests/UnitTests/SubscriberAttributes/__Snapshots__/BackendSubscriberAttributesTests/iOS15-testPostSubscriberAttributesCallsCompletionWithErrorInBadRequestCase.1.json b/Tests/UnitTests/SubscriberAttributes/__Snapshots__/BackendSubscriberAttributesTests/iOS15-testPostSubscriberAttributesCallsCompletionWithErrorInBadRequestCase.1.json
index 9c20fe3176..560035efeb 100644
--- a/Tests/UnitTests/SubscriberAttributes/__Snapshots__/BackendSubscriberAttributesTests/iOS15-testPostSubscriberAttributesCallsCompletionWithErrorInBadRequestCase.1.json
+++ b/Tests/UnitTests/SubscriberAttributes/__Snapshots__/BackendSubscriberAttributesTests/iOS15-testPostSubscriberAttributesCallsCompletionWithErrorInBadRequestCase.1.json
@@ -14,7 +14,7 @@
     "X-Platform-Flavor" : "Unity",
     "X-Platform-Flavor-Version" : "2.3.3",
     "X-Platform-Version" : "Version 17.0.0 (Build 21A342)",
-    "X-Preferred-Locales" : "en",
+    "X-Preferred-Locales" : "en_US",
     "X-Retry-Count" : "0",
     "X-Storefront" : "USA",
     "X-StoreKit-Version" : "1",
diff --git a/Tests/UnitTests/SubscriberAttributes/__Snapshots__/BackendSubscriberAttributesTests/iOS15-testPostSubscriberAttributesSendsAttributesErrorsIfAny.1.json b/Tests/UnitTests/SubscriberAttributes/__Snapshots__/BackendSubscriberAttributesTests/iOS15-testPostSubscriberAttributesSendsAttributesErrorsIfAny.1.json
index 9c20fe3176..560035efeb 100644
--- a/Tests/UnitTests/SubscriberAttributes/__Snapshots__/BackendSubscriberAttributesTests/iOS15-testPostSubscriberAttributesSendsAttributesErrorsIfAny.1.json
+++ b/Tests/UnitTests/SubscriberAttributes/__Snapshots__/BackendSubscriberAttributesTests/iOS15-testPostSubscriberAttributesSendsAttributesErrorsIfAny.1.json
@@ -14,7 +14,7 @@
     "X-Platform-Flavor" : "Unity",
     "X-Platform-Flavor-Version" : "2.3.3",
     "X-Platform-Version" : "Version 17.0.0 (Build 21A342)",
-    "X-Preferred-Locales" : "en",
+    "X-Preferred-Locales" : "en_US",
     "X-Retry-Count" : "0",
     "X-Storefront" : "USA",
     "X-StoreKit-Version" : "1",
diff --git a/Tests/UnitTests/SubscriberAttributes/__Snapshots__/BackendSubscriberAttributesTests/iOS15-testPostSubscriberAttributesSendsRightParameters.1.json b/Tests/UnitTests/SubscriberAttributes/__Snapshots__/BackendSubscriberAttributesTests/iOS15-testPostSubscriberAttributesSendsRightParameters.1.json
index 9c20fe3176..560035efeb 100644
--- a/Tests/UnitTests/SubscriberAttributes/__Snapshots__/BackendSubscriberAttributesTests/iOS15-testPostSubscriberAttributesSendsRightParameters.1.json
+++ b/Tests/UnitTests/SubscriberAttributes/__Snapshots__/BackendSubscriberAttributesTests/iOS15-testPostSubscriberAttributesSendsRightParameters.1.json
@@ -14,7 +14,7 @@
     "X-Platform-Flavor" : "Unity",
     "X-Platform-Flavor-Version" : "2.3.3",
     "X-Platform-Version" : "Version 17.0.0 (Build 21A342)",
-    "X-Preferred-Locales" : "en",
+    "X-Preferred-Locales" : "en_US",
     "X-Retry-Count" : "0",
     "X-Storefront" : "USA",
     "X-StoreKit-Version" : "1",