The fact that FileVault2 is limited to English or similar is probably due to a) this being simpler for Apple to develop and store the values of, and b) there probably not being a means in the FileVault2 login screen to select alternative keyboard layouts.
All of the above would I am sure be possible to fix but probably was not a top priority initially.
I would suggest posting feedback here http://www.apple.com/feedback/macosx.html suggesting this as a new feature. I am all in favour of getting Apple to make their products more friendly to international markets even if it is of no direct relevance to myself. I am particularly hacked off by Microsoft still not supporting right-to-left languages like Hebrew and Arabic in Word for Mac with no likelihood this will change even in the forthcoming Office 2014 for Mac.
Note to Apple and Microsoft - over 50% of sales of Apple products are outside the USA and this proportion continues to grow.
PS. Apple where is iTunes Radio for the UK???