I confirm this happens in the 1.3.0-rc1 head
However, in current master, it's working fine.

However, in current master, it's working fine.
Because we got localisation updates from https://translatewiki.net https://github.com/mantisbt/mantisbt/commit/ceb94b39641a755742c07482c4bff0ae0521eb97

This message is normal.

The 'filter_match_type' language string was removed from English file in MantisBT master 48593d34 but other language files were not updated as we rely on TranslateWiki for that.

I believe the fix for this issue is to remove the check from the script. It is perfectly normal to have strings in English, but not in other language and vice versa.

I've discussed it with @dregad offline and he looked up the change which was added in commit 41a4ea5 by grangeway.

To be clear:

It is perfectly normal to have strings in English, but not in other language

Yes

and vice versa.

That is not a normal situation.

I don't consider it an issue, because in theory this should not happen (since TranslateWiki generates the foreign language files based on the English one).

In that case, the error highlighted the fact that we did not update the translations prior to releasing rc1.