Introduce language suggester for monolingual

This is an alternative implementation of #93.

Please note that this patch is only a first step. Some code should be moved from `ExpertExtender.LanguageSelector.js` to this new file. But I don't want to make this to complicated. More refactoring can easily be done after this is reviewed and merged.

Questions:
* I'm not sure what the current best naming practice is. I modeled `suggestLanguage` after `suggestCommons`.
* It's probably better to use data objects instead of strings, similar to `pagesuggester` for example.