Hi, I must confess I haven't followed all the nooks and crannies of this subject, but when I browsed through the latest batch of contributions, I noticed that one simple (=crude) workaround hasn't been mentioned; Indirect buffers.
If one uses one indirect buffer per language, it should be possible to select a separate flyspell language for each buffer. Jumping between buffers/windows is perhaps less of a hassle than constantly switching spell checking languages. I suspect the ambitions of the general list member is higher than that, but this workaround could ease the pain for some of us. Cheers Rasmus