Make sure to pass underscore as separator

cmd/fyne/internal/commands/package-darwin.go

@@ -24,6 +24,14 @@ type darwinData struct {
 	Languages     []string
 }
 
+func darwinLangs(langs []string) []string {
+	r := make([]string, len(langs))
+	for n, lang := range langs {
+		r[n] = strings.Replace(lang, "-", "_", 1)
+	}
+	return r
+}
+
 func (p *Packager) packageDarwin() (err error) {
 	appDir := util.EnsureSubDir(p.dir, p.Name+".app")
 	exeName := filepath.Base(p.exe)
@@ -41,7 +49,7 @@ func (p *Packager) packageDarwin() (err error) {
 	}()
 
 	tplData := darwinData{Name: p.Name, ExeName: exeName, AppID: p.AppID, Version: p.AppVersion, Build: p.AppBuild,
-		Category: strings.ToLower(p.category), Languages: p.langs}
+		Category: strings.ToLower(p.category), Languages: darwinLangs(p.langs)}
 	if err := templates.InfoPlistDarwin.Execute(infoFile, tplData); err != nil {
 		return fmt.Errorf("failed to write plist template: %w", err)
 	}

cmd/fyne/internal/commands/package-darwin_test.go

@@ -0,0 +1,15 @@
+package commands
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestDarwinLangs(t *testing.T) {
+	langs := darwinLangs([]string{"en-GB", "de-CH"})
+
+	assert.Equal(t, 2, len(langs))
+	assert.Equal(t, "en_GB", langs[0])
+	assert.Equal(t, "de_CH", langs[1])
+}