Study languages if you enjoy them! Law can be a tough slug at times, and having something fun in your arts degree can provide a nice break. (Not that languages are easy - I did French and German and it was hard, hard work, but for the large part enjoyable)
Knowing languages is definitely a plus for diplomacy-related careers - especially Asian languages (although the level of fluency you'll achieve starting from scratch at uni won't be sufficient - have you considered going on exchange?)
I have to check the DFAT media releases page every week at work, and newly appointed diplomats (High Commissioners and such) to foreign countries will often have something like a masters in [language relevant to the country they have been seconded to]. Interestingly, a lot of them DON'T have law degrees but rather things like Masters in International Relations and such.
Also I hear the UN graduate programs require you to know at least 3 languages (from what I've heard anyway - I haven't checked it out myself but a girl from work is interested in it)
edit: anecdote from someone I know who is now a lawyer at a top tier law firm whose name I forget for the moment - one of her fellow graduate junior lawyers said the interviewing partners were highly interested in the activities that had absolutely nothing to do with law - e.g. the fact that she was a national-level ballet dancer and the fact that she worked in retail for a long time. It showed that she had, well, a life outside of law, and an ability to work in a team yadayadayada. Something for you to think about as well - law firms don't solely care about your degree!