Language selection fixes (#2917)

* Fix duplicated language entries from system and app language sets.
Closes #2900

* Prefer modern language codes.
Closes #2903

* Synchronize per-account default posting language with server.
Closes #2902

* Allow users to post in languages android doesn't know about yet (e.g. toki pona)

* Always put the preselected language at the top of the list

app/src/main/java/com/keylesspalace/tusky/util/LocaleExtensions.kt

@@ -0,0 +1,11 @@
+package com.keylesspalace.tusky.util
+
+import java.util.Locale
+
+// When a language code has changed, `language` *explicitly* returns the obsolete version,
+// but `toLanguageTag()` uses the current version
+// https://developer.android.com/reference/java/util/Locale#getLanguage()
+val Locale.modernLanguageCode: String
+    get() {
+        return this.toLanguageTag().split('-', limit = 2)[0]
+    }