It would seem that the latest version of MediaInfo (V24) is often returning different language strings than previous versions, and now MeGUI does not recognise the strings, causing One-Click to use the default language instead.
I was using V18 before which would return something like this:
----[Information] Language: ko
----[Information] LanguageString: Korean
V24 now may return something like this instead:
-[Information] Language: ko-KR
-[Information] LanguageString: Korean (KR)
-[Warning] [2024/07/11 19:30:41] The language information "ko-KR/Korean (KR)" is unknown. The default MeGUI language has been selected instead.