I don’t think the problem is related to a specific translation.
When a post is hidden because of flags from the community a personal message is sent to the author. This message contains a reason. For example:
These reasons are used when the message is created.
The code which adds this reason to the message uses the post_action_type
That works for the default flag reasons. For example, flag_reasons.spam, as seen in the screenshot above.
The problem is that there is no such string for custom flag reasons. For example, if you create a “Testing” reason and enable Auto hide flagged content.
Then flag_reasons.custom_testing is used. However, this reason does not exist. The language doesn’t matter; it does not even exist in English because it’s a custom flag.
And the same reason is also used when the post is deleted (see screenshot in RGJ’s post)
By the way for “responders” the illegal reason is missing too

