[I_hate_popcorn]

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Is there any way to accomplish what I want with the addon as it is or maybe you'd consider adding support for language Unknown (Either by allowing to pick it as a language or possibly adding an option to treat all unknown language as a certain language)?
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Hello and sorry for a late answer I have been running behind many things since 2025 

As you saw it, currently LPM just ignores Undefined/Unknown tracks (both audio or subs).
Only chance you have them being activated is when no other tracks are present matching your LPM rules, then LPM defaults back to KODI behaviour who will activate them if they are default, or first in lists for example. Note that external subtitle tracks are always coming first in list versus internal container tracks.

Between your 2 proposals I tend to prefer the first one : create a additional "und"/"unk" language in the main table. No logic to be changed: just for users to decide to start using this in their rules as you propose.

I will add that to my to-do list for the future release.

No problem on the late (it really isn't) response. I'm happy to hear this makes sense and hope it'll not be too much trouble to implement.
Here's an excerpt of the kodi log regarding LPM reporting video properties when I tried playing back a movie and it picking the embedded English subs instead of the external "und"/"unk" .srt in my native tongue. I hope this helps. As you can see the 1st subtitle (index 0) has an empty language and the name is "(חיצוני)" actually means "(external)". It's unfortunate that Kodi's API localizes this bit of info, for some reason, and I hope it'll not present additional challenges.
If there's anything else I can do to assist let me know.

Code:

2025-02-02 14:29:04.731 T:25486   debug <general>: [Language Preference Manager]: Getting video properties
2025-02-02 14:29:04.735 T:25486   debug <general>: [Language Preference Manager]: {'id': 1, 'jsonrpc': '2.0', 'result': {'audiostreams': [{'bitrate': 1024000, 'channels': 6, 'codec': 'eac3', 'index': 0, 'isdefault': True, 'isimpaired': False, 'isoriginal': False, 'language': 'eng', 'name': 'DD+ 5.1(side)', 'samplerate': 0}], 'currentaudiostream': {'bitrate': 1024000, 'channels': 6, 'codec': 'eac3', 'index': 0, 'isdefault': True, 'isimpaired': False, 'isoriginal': False, 'language': 'eng', 'name': 'DD+ 5.1(side)', 'samplerate': 0}, 'currentsubtitle': {'index': 1, 'isdefault': False, 'isforced': False, 'isimpaired': False, 'language': 'eng', 'name': ''}, 'subtitleenabled': True, 'subtitles': [{'index': 0, 'isdefault': False, 'isforced': False, 'isimpaired': False, 'language': '', 'name': '(חיצוני)'}, {'index': 1, 'isdefault': False, 'isforced': False, 'isimpaired': False, 'language': 'eng', 'name': ''}, {'index': 2, 'isdefault': False, 'isforced': False, 'isimpaired': True, 'language': 'eng', 'name': 'SDH'}, {'index': 3, 'isdefault': False, 'isforced': False, 'isimpaired': False, 'language': 'bul', 'name': ''}, {'index': 4, 'isdefault': False, 'isforced': False, 'isimpaired': False, 'language': 'fre', 'name': ''}, {'index': 5, 'isdefault': False, 'isforced': False, 'isimpaired': False, 'language': 'ger', 'name': ''}, {'index': 6, 'isdefault': False, 'isforced': False, 'isimpaired': False, 'language': 'ind', 'name': ''}, {'index': 7, 'isdefault': False, 'isforced': False, 'isimpaired': False, 'language': 'por', 'name': 'Brazilian'}, {'index': 8, 'isdefault': False, 'isforced': False, 'isimpaired': False, 'language': 'spa', 'name': ''}, {'index': 9, 'isdefault': False, 'isforced': False, 'isimpaired': False, 'language': 'tur', 'name': ''}]}}