How to approach Multilingual Text Classification?
In a similar situation, -after trying some alternatives- I had to build a language classifier in front of all learning and classification steps.

That is, for learning:


Detect the language of the input (say, an enumeration like "DE", "EN", etc.)
Apply language specific stemming to the words of the input.
Prepend words in the learning phase with the language identifier (i.e. "de_du", "en_you")
Use these words in a single training model.


In classification phase, use the same steps.