In this case, the OP author wasn’t Kbin, but the language of the main post was set as Undetermined (or not set). So replying to it had the same issue as replying to Kbin users. I wrote up a bit more detail in my reply to the OP.
This post is not showing a language, so the error in this case might be legitimate. The error can also be relate to a bug in Lemmy.
The first thing to check is if you have a language set in your Lemmy profile settings. Open a web browser and go to your Lemmy server (not Voyager). Login, click you name in the top right > Settings.
Set your Interface Language to your main spoken language.
Unrelated to this issue, but while you’re there, you may also want to check that the Languages have Undetermined, and all other languages you want to see on Lemmy selected. If you’re using a Desktop, you may need to Ctrl+Click to select multiple entries.
If your Interface Language was already set, the issue you have replying to comments can be related to the language of the parent comment or post. I’ve found Kbin comments are showing as Undetermined language on Lemmy, despite having a language set on Kbin.
When replying to an Undetermined language comment or post, Lemmy seems to try to use the same language and generates an error in the API/app (if you try to reply directly in Lemmy via browser it just shows a spinning loading button when you hit Post).
A work around is to reply to the comment directly on your Lemmy server via a web browser (not in Voyager), and selecting a language other than Undermined on the dropdown near the below the text field of your comment.
This post is a perfect example of this bug, I had the error trying to reply to it via Voyager, but could reply if I logged in, and selected English language on my comment.
The parent comment issue is with Lemmy rather than Voyager, but Voyager are tracking the issue here.
It’s related to the language of the parent comment. I’ve found Kbin comments are showing as Undetermined on Lemmy, despite having a language set on Kbin.
When replying to an Undetermined language comment, Lemmy seems to try to use the same language and generates an error (although the error is silent in a browser rather than API/app).
A work around is to reply to the comment directly on your Lemmy’s instance via a web browser (not in Voyager), and selecting a language other than Undermined on your comment.
The issue is with Lemmy rather than Voyager, but Voyager are tracking the issue here.
Off topic, but I noticed that I had issues replying to your comment in my Lemmy app (Voyager). I’m trying to work out why.
Logging in via a web browser, I can see that the post and parent comments are tagged as English, but your comment language is Undetermined. If I tag my post as language Undermined, my reply doesn’t get sent, just shows a loading animation. If I change it to English, it posts, but I’m not sure if that will display for you. Please let me know if you see this message.
This could be preventing some people engaging with your posts. In Kbin settings, can you define the language of your posts?
In this case, the OP author wasn’t Kbin, but the language of the main post was set as Undetermined (or not set). So replying to it had the same issue as replying to Kbin users. I wrote up a bit more detail in my reply to the OP.