Must a master's thesis fall within one's advisor's area of expertise?
Whether an advisor will agree to supervise a master's thesis outside of their area of expertise depends on the advisor.  It may also vary between fields or universities, but I believe the personal variance will be much higher, so the best way to find out is to ask a potential advisor.

As for whether it's a good idea, I see three issues.


You will not receive expert advice in your chosen area.  Of course, there's a trade-off here: would you rather study what you love best without expert advice, or get better advising on another topic?
You may receive actively bad advice.  To avoid this difficulty, it's important for you to read extensively and to choose an advisor who is open minded and flexible.
Your advisor may not write an enthusiastic letter of recommendation for you.  Sometimes an advisor will let a student choose to do whatever they want, but when the advisor recommends the student, it becomes clear that the advisor is uninterested in the student's work and unimpressed by it.  I've seen this cause trouble for several people, so I strongly recommend trying to find an advisor who is actually enthusiastic about your topic (even if it's not what they work on).


In your specific case, choosing to study second language acquisition in a language nobody on the faculty speaks sounds like only a mild concern (assuming you speak it fluently or have access to excellent informants), since I imagine much of the advising would be about research methodology or language acquisition theories, rather than language-specific issues.  However, I don't know enough about this area to say for sure.