Other languages XML files are included but how to make component use them?

I have downloaded and copied in the corresponding place a language XML file.
All my users need to see instructions and prompts in that language. The language of the reports is all the same, in that language, too.
So, I simply need the screen to show all prompts and labels in that particular language.
The XML file for that particular language IS included. However, there is no CLEAR way as to how to use it.
Can anybody please explain?
For the time being we simply need our app to use the same language for all reports. We do not need one language per report.