Is congregational prayer valid if the imam errs in pronouncing letters, such as pronouncing "dhāl" as "zāy," for instance, by saying "a‘ūzu birabbin-nās" instead of "a‘ūdhu birabbin-nās"?

The prayer of someone who substitutes one letter for another is valid for himself if he cannot learn. However, if he can learn but does not, then his prayer is invalid.

If he leads someone who recites correctly, the prayer of the follower is invalid because he is led by someone whom it is impermissible to follow.

This applies if the grammatical error (lahn) is in Al-Fatihah. As for errors outside of Al-Fatihah, his prayer and the prayer of those behind him are valid, unless he intentionally substitutes a letter, in which case both their prayers become invalid.