Can someone help me how to set it up that comments I write are kept in “undetermined”?
You do not want this. The language flags exist for a reason, and provided the community you are posting/commenting in is configured correctly, things should work right on their own.
This is because “default” is determined by the languages set for the community you are posting/commenting in, not your account. (Though some clients have had trouble respecting this).
If “undetermined” was your default, but that weren’t enabled for the community, the comment/post wouldn’t go through. Clients that don’t respect this run into errors when users try to post or comment. Hence, unless you manually set the language of your post/comment, what it ends up as depends on where you are posting/commenting.
It’s not something you can configure as a user (unless you mod a community).
It’s worth noting that you can select multiple languages in your account settings, to make sure you’ll see any and all relevant content.
Additonally, if you post everything with “undetermined” that still means that people who don’t have that “language” enabled, can’t see your content. It’s not a “everyone will see this” setting.
There is no “unflagged”. “Undetermined” works exactly as if it was just another language, to the point that if a community doesn’t have it allowed, you cannot make posts or comments that are “undetermined”.
Which they shouldn’t, because then content might not get correctly tagged. Because, again, “undetermined” is just another language and can prevent people from seeing content in the same way.
When you don’t select a language when posting or commenting, the lemmy server picks for you, based on how the community is set. When configured correctly, your posts will automatically get tagged with the right language.
If not, then the community mods have not set things up correctly.