Changing your language affects gameplay in several ways.
First of all, questions, answers and all other text in QuizUp will be in the language you have chosen.
You will see a localized list of topics which includes all topics that have been translated to your chosen language, as well as special topics only available in that language.
In translated topics you can play against players using all other languages - they will be answering the same questions as you, but in their chosen language. In topics exclusively available in your language you will be playing users that have chosen the same language as you.
However, it is possible to challenge a user of any language in the local topics, but if they accept they will have to play the match in the language of the local topic. For example, if you have chosen German, and challenge an English speaker to a match in a German topic, you will both play the match in German.