Translations: don't show language flags in foldertree when only 1 language is used in the repository

We decided to only specify a language on root folders - although it's always possible to set a different language on a deeper level language using the console.
Also, we'll drop the necessity for having "non-translated folders", so every rootfolder will have a language set when created using the Add Root Folder dialog.
This means that every rootfolder will show a language flag, which is overkill when only 1 language is used in the repository.

Improvement: only show language flag icons in the foldertree when there's more than 1 language used in the repository.

It would be good enough functionality to simplify this to "don't show flag icons when all folders on the same level use the same language", if this simplifies things during development (but this is not a requirement).