According to various online sources (1, 2), in the letter sequence vowel + ill the ll should be pronounced as the English "y", or [j] in the IPA. This rule seems to apply in general, e.g. in the word "cuillère": [kɥi'jɛʁ].
According to the German wikipedia however, the Guillain–Barré syndrome, named after the Rouen-born neurologist Georges Guillain, is to be pronounced [ɡilɛ̃ baˈʁeː zʏnˈdʀoːm] with ll being pronounced as [l] instead of [j] (note that syndrome is supposed to be pronounced in German). The German Wikipedia explicitly notes that the pronunciation [ɡiˈjɛ̃ baˈʁeː zʏnˈdʀoːm] with ll being pronounced as [j] is frequent, but wrong ("häufig, aber fälschlich").
The English Wikipedia on the other hand suggests the pronunciation [ɡijɛ̃] for the name Guillain in the respective article on Georges Guillain with ll being pronounced as [j]. The pronunciation is unsourced in both the German and the English articles.
In this French youtube video (mark 00:03) on the syndrome the name seems to be pronounced as [ɡilɛ̃] concurrent with the German article.
Due to my poor capabilities in the French language, I couldn't think of any other words with the combination vowel + ill + a apart from the name Guillaume, in which ll is obviously pronounced [j].
I would highly appreciate if someone could:
- Confirm that the pronunciation [ɡilɛ̃] is correct/common.
- Clarify whether an actual rule for this pronunciation exists, e.g. that in the combination vowel + ill + ai + consonant (as in Guillain), [l] is correct, whereas in other combinations with vowel + ill + a as in Guillaume [j] is correct. Or, perhaps, whether the question comes down to idiosyncratic pronunciation of proper nouns, which is bound to history/tradition rather than rule books.