Language settings based on user selection

I need to display the labels and other visible texts of my WD app based on the language choosen by the user at the time of accessing the app.

In the index page of the app,the user is supposed to choose a language (out of two,say english and German).Depending on what he chooses,the labels of the next screen will appear in that language.

Can anybody tell how to do this.?How to make my app read the particular .xlf file depending on the user selection?

regds,

Bhupesh