auto-dictionary-mode tries to guess the buffer's text language and adjusts flyspell automatically. Currently English, German, French, Spanish, Swedish, Slovenian, Hungarian, Romanian and Portuguese are somewhat supported. Additional languages are easily added. (Please contact me, if you'd like to contribute.)
It is also possible to insert text according to the current dictionary and have it updated automatically when it changes. This is great for your text templates and signatures.
This project is now available on github.