Match language fallback mapping

Problem/Motivation

Sometimes the local language contains ISO recommended extensions like "en-CH" for a swiss variation of english.
This is needed, because "en" is already in use and we want to define a derivative.

However, en-CH doesn't map with translators "en" as it only does exact matching... So translations don't work out of the box.

Proposed resolution

Then doing the default mapping, if no exact match "en" is found, try partial matches.
The target en-CH would then match en.

This is how language fallback mapping should work to best map ISO based naming out of the box.
Separator should be "-" and supporting one dash is enough.

Remaining tasks

User interface changes

API changes