Fix/441 Fail if no language is set (#461)

## Description, Context and related Issue

Fixes #441

## Types of changes
- Bug fix (non-breaking change which fixes an issue)
- 
## Checklist
- I have performed a self-review of my code

add/controller.xql

@@ -35,7 +35,6 @@ return
         (: forward index.html to index.xql :)
         <dispatch xmlns="http://exist.sourceforge.net/NS/exist">
             <forward url="index.xql">
-                <set-header name="Set-Cookie" value="edirom-language={$langVal}" />
                 <add-parameter name="lang" value="{$langVal}"/>
             </forward>
         </dispatch>

add/data/xqm/eutil.xqm

@@ -281,15 +281,22 @@ declare function eutil:getLanguageString($key as xs:string, $values as xs:string
  : @param $edition The current edition URI
  : @return The preference value
  :)
-declare function eutil:getPreference($key as xs:string, $edition as xs:string?) as xs:string {
+declare function eutil:getPreference($key as xs:string, $edition as xs:string?) as xs:string? {
 
-    let $preferencesFile := 
+    (: Try to load a custom preferences file :)
+    let $prefFileCustom := 
         try { doc(edition:getPreferencesURI($edition)) }
-        catch * { util:log-system-out('Failed to load preferences') }
-
+        catch * { util:log-system-out('Failed to load the custom preferences file') }
+    
     return
-        $preferencesFile//entry[@key = $key]/@value => string()
-
+        (: If there is a value for the key in the custom preferences file :)
+        if($prefFileCustom//entry/@key = $key) then
+            $prefFileCustom//entry[@key = $key]/@value => string()
+        (: If not, take the value for the key in the default preferences file :)
+        else
+            try { doc($edition:default-prefs-location)//entry[@key = $key]/@value => string() }
+            (: If the key is not in the default file, then there should be an error :)
+            catch * { util:log-system-out(concat('Failed to find the key `', $key, '` in default preferences file')) }
 };
 
 (:~
@@ -299,16 +306,13 @@ declare function eutil:getPreference($key as xs:string, $edition as xs:string?)
  : @return The language key
  :)
 declare function eutil:getLanguage($edition as xs:string?) as xs:string {
-
+    
     if (request:get-parameter("lang", "") != "") then
         request:get-parameter("lang", "")
-
-    else if(request:get-cookie-names() = 'edirom-language') then
-        request:get-cookie-value('edirom-language')
-
-    else
+    else if(eutil:getPreference('application_language', edition:findEdition($edition))) then
         eutil:getPreference('application_language', edition:findEdition($edition))
-
+    else    
+        'de'
 };
 
 (:~