Prepare for selecting languages

UI language is stored in user settings for authenticated users;
otherwise is only stored in the current model (not persisted).

modules/webapp/src/main/elm/UiLanguage.elm

@@ -0,0 +1,16 @@
+module UiLanguage exposing
+    ( UiLanguage(..)
+    , all
+    )
+
+
+type UiLanguage
+    = English
+    | German
+
+
+all : List UiLanguage
+all =
+    [ English
+    , German
+    ]