Fix: fix #1506, language-URL-parameter works again and is properly documented

Logic/State/UserRelatedState.ts

@@ -92,7 +92,7 @@ export default class UserRelatedState {
             this.osmConnection.GetLongPreference("identity", "mangrove")
         )
 
-        this.InitializeLanguage(availableLanguages)
+        this.language.addCallbackAndRunD((language) => Locale.language.setData(language))
 
         this.installedUserThemes = this.InitInstalledUserThemes()
 
@@ -178,28 +178,6 @@ export default class UserRelatedState {
             )
         }
     }
-
-    private InitializeLanguage(availableLanguages?: string[]) {
-        this.language.addCallbackAndRunD((language) => Locale.language.setData(language))
-        Locale.language.addCallback((currentLanguage) => {
-            if (Locale.showLinkToWeblate.data) {
-                return true // Disable auto switching as we are in translators mode
-            }
-            if (availableLanguages?.indexOf(currentLanguage) < 0) {
-                console.log(
-                    "Resetting language to",
-                    availableLanguages[0],
-                    "as",
-                    currentLanguage,
-                    " is unsupported"
-                )
-                // The current language is not supported -> switch to a supported one
-                Locale.language.setData(availableLanguages[0])
-            }
-        })
-        Locale.language.ping()
-    }
-
     private InitInstalledUserThemes(): Store<string[]> {
         const prefix = "mapcomplete-unofficial-theme-"
         const postfix = "-combined-length"

Logic/Web/QueryParameters.ts

@@ -54,6 +54,7 @@ export class QueryParameters {
     }
 
     public static wasInitialized(key: string): boolean {
+        this.init()
         return QueryParameters._wasInitialized.has(key)
     }
 
@@ -76,11 +77,10 @@ export class QueryParameters {
         if (window?.location?.search) {
             const params = window.location.search.substr(1).split("&")
             for (const param of params) {
-                const kv = param.split("=")
+                const [key, value] = param.split("=")
-                const key = decodeURIComponent(kv[0])
                 QueryParameters.addOrder(key)
                 QueryParameters._wasInitialized.add(key)
-                const v = decodeURIComponent(kv[1])
+                const v = decodeURIComponent(value)
                 const source = new UIEventSource<string>(v)
                 source.addCallback(() => QueryParameters.Serialize())
                 QueryParameters.knownSources[key] = source
@@ -131,4 +131,5 @@ export class QueryParameters {
         QueryParameters._wasInitialized.clear()
         QueryParameters.order = []
     }
+
 }

UI/i18n/Locale.ts

@@ -11,26 +11,50 @@ export default class Locale {
     public static showLinkOnMobile: UIEventSource<boolean> = new UIEventSource<boolean>(false)
     public static language: UIEventSource<string> = Locale.setup()
 
+    /**
+     * Creates the UIEventSource containing the identifier of the current language
+     *
+     * If the QueryParameter 'language' is set, this query parameter will be used as backing source value
+     * If not set, a localStorageSource will be used. This will use the navigator language by default
+     *
+     * Note that other parts of the code (most notably the UserRelatedState) might sync language selection with OSM.
+     *
+     *
+     * @private
+     */
     private static setup() {
+
+        let source: UIEventSource<string>
+
+        if (QueryParameters.wasInitialized("language") || Utils.runningFromConsole) {
+            console.log("Language was initialized via URL-parameter - using the URL parameter as store instead of local storage", QueryParameters.wasInitialized("language"))
+            source = QueryParameters.GetQueryParameter(
+                "language",
+                undefined,
+                ["The language to display MapComplete in.",
+                    "The user display language is determined in the following order:",
+                    "- If the user did log in and did set their language before with MapComplete, use this language",
+                    "- If the user visited MapComplete before and did change their language, use the language as set by this URL-parameter. This will _disable_ saving the language to localStorage in case a non-logged-in user changes their language",
+                    "- Use the navigator-language (if available)",
+                    "- Use English",
+                    "",
+                    "Translations are never complete. If a translation in a certain language is missing, English is used as fallback."].join("\n"),
+            )
+        } else {
             let browserLanguage = "en"
             if (typeof navigator !== "undefined") {
                 browserLanguage = navigator.languages?.[0] ?? navigator.language ?? "en"
+                console.log("Browser language is", browserLanguage)
             }
-        const source = LocalStorageSource.Get("language", browserLanguage)
+            source = LocalStorageSource.Get("language", browserLanguage)
+        }
+
         if (!Utils.runningFromConsole) {
             // @ts-ignore
             window.setLanguage = function (language: string) {
                 source.setData(language)
             }
         }
-        if (QueryParameters.wasInitialized("language")) {
-            const qp = QueryParameters.GetQueryParameter(
-                "language",
-                undefined,
-                "The language to display mapcomplete in. Will be ignored in case a logged-in-user did set their language before. If the specified language does not exist, it will default to the first language in the theme."
-            )
-            Locale.language.setData(qp.data)
-        }
 
         QueryParameters.GetBooleanQueryParameter(
             "fs-translation-mode",