How important is the language of the bachelor thesis?
It should have no effect, whatever on your future. You can fulfill all of the requirements of your university, but also, when you have the time, provide a translation of your thesis for others to peruse. You can, in theory, produce an English language publication based on the thesis in another language. 

You need to demonstrate some English proficiency for study in some countries, but it needn't be through your official thesis. There are many other ways. In fact, a translation, done by yourself, might be an especially good way. 

Yes, my doctorate is in mathematics, though I worked mostly in CS. Your mathematics will be much more important than whether the thesis is in one language or another.