I need a single C5.7 installation and this language-structure:
What is the best strategy to use the multilanguage-functions?
It's unfortunately not possible to add multiple locations to the same language.
I think the practical way is to have same number installations as subdirectories in your requirements.
This is a key-requirement for my project...
At the time before the multilingual features were integrated with the core, I was actually trying to raise this exact point in the discussion but it didn't get much attention:
I think the multilingual features should allow adding multiple "multilingual roots" to the site under which you have the same limitations that you currently have for the whole site, i.e. each language can only exist once under each "multilingual root".
If you're planning to do some work on this, consider also modifying the core and providing the modified code back into the core as a pull request. I think the multilingual features could be much more flexible than they currently are.
I also raised the same point about "language roots" here:
If you comment there, it might get more attention from the people involved in the development of the multilingual features.