How to treat multi language users?

In some areas of the world people are aware of multiple languages. For example in China there is “Simplified” and “Traditional” where two different language codes exist. Something else also exist for Norwegian. I also assume that Espanyol/Portuguese/Esperanto is such a case.

How do they setup their system (GNU Linux, Windows, OS X, etc)? Can they setup multiple languages?

How do you treat situations like this in your translation pipeline as a developer?