There is a number of non-english speakers logging in to StackOverflow (including me). There also is some rant about badly formulated questions, when the guys barely know the language adopted here.
If one of the aims is to satisfy the former and to decrease the latter, then the captions should be as polished as possible. I cannot judge whether "log in" form is better than "login", it's English-speaking admins who should. But it's easy to imagine that the caption was incorrect for too long because those who know the language, know where the mistake is, and those who don't, think that it's the correct way to spell it.
Fix it, please, if it's incorrect. I'm not really sure, but...