That one is showing as Undetermined as well. I setup a Kbin account and had the same behaviour, comment was set to English in Kbin, but would appear as Undetermined on Lemmy.

The issue with replying from Lemmy seems to occur when Communities have set their Language to only English (or at least not Undetermined). For some reason the Kbin supposedly Undetermined messages are allowed to post, but if anyone on Lemmy tries to reply to it, Lemmy will try to use the same language, which will result in a silent error (but visible in Developer Tools).

Again, manually changing my reply on Lemmy to specifically use English works.

In conclusion there’s probably nothing you can do, but there is an issue for Lemmy language issue here.