The language tag has been set on the video stream, not the audio stream.
Tagging the video with a language is only useful for indicating the language of either the title card and credit roll, or burned-in subtitles.
But if it is the language of the spoken language you are looking for, you need to tag the audio stream(s).
Looking a little bit closer now, I have another idea:
Get MKVtoolnixGUI and drag your file into it.
You will probably see something like this:
The “Global Tags” and “Tags” I’ve marked in red, can contain contradictory information to the traditional MKV tags which appear in the “Language” column above.
It is almost always best to remove these tags completely (simply untick the check boxes in front of their lines) and remux the file.
You can even create a global preference in MKVToolnix to do this automatically each time.
Handbrake is one of the apps which insist in writing these tags, even though the information in them is usually garbage.
