what languages are used to make open source software?

english

You could, but the vast majority of open source software is written in either C or C++.

any language is ok for opensource, but not for “free” (free as in free speech) software: they are a rather different thing. you, as the “owner” (creator) of the source, have the power to make it available under certain conditions. to meet the requirement of free (free as in free speech) software, anyway, probabily there are languages that must be preferred; e.g. the .NET framework could hide some traps and making a software depending on it could mean to make it “less free” (e.g. if it runs on DotGNU, it’s ok, but there are classes that can’t be implemented in DotGNU, because of copyrights… so Mono may appear “better”, but it is a compromise, so the sw freedom is at risk); this speech applies if you’ve meant “free” software, but opensource software does not imply “free” in the FSF’s sense. a lot of “free” and opensource sw are written in C/C++, but langs like python are becoming popular too (not counting php mainly in the web)

I suppose anyway that for the GSC ou can use your preferred lang without worrying too much. just check what *they* say, the rules they decided!!

It can be written in any language. Surely most (if not all) languages are open source? The most0 famous, in terms of open source/prtability etc. is Unix.

This is what the FAQ says about languages:

3.What language(s) should a student program in?
Talk with your mentoring organization about this and other technical style questions. Of course, the Python people will prefer Python submissions, and so on. Students should let us know in their applications what languages they’re thinking about using.