Source Selector: Set default languages

* Adds method to set default languages when rendering source selector
** Uses localStorage target language if it exists
** Defaults target language to wiki content language
** Uses localStorage source language if exists and valid for target language
** Defaults source language to first valid source language for target language
* Adds convenience methods for setting source and target languages
* Adds method for retrieving valid source and target languages
* Adds method for validating source language against target language
* Modifies method for validating target language against source langauge
* Updates change handlers for source language and target language

This patch also solves the issue in T76700.

Bug: T76832
Change-Id: I9c57c9d6a8dfc7bfa8f36336ef513af3a9254254