Improved MKV subtitle extraction: detect languages, prefer Chinese, and prioritize supplied subtitles

The previous update automatically extracted subtitles from MKV files and burned them into videos. Feedback was positive, but some users found English subtitles in the transcoded MKV output. Investigation showed these MKV files contained English, Chinese and Spanish subtitles, with English as the default. Extraction selected that track, so the resulting mp4 naturally contained English. After reviewing the documentation, I found a way to identify subtitle languages, leading to this update. During development, I also found a priority issue: a user could place a matching ASS subtitle alongside an MKV in the import folder, but the extraction process overwrote the user's file with the MKV's embedded subtitles. This update fixes that priority as well.

Changelog

• One: During MKV subtitle extraction, detect whether Chinese subtitles exist and extract them preferentially. If none are found, extract the first subtitle track by default.
• Two: Matching srt or ass files in the import folder have the highest priority. If present, subtitles are not extracted even when the mkv contains them.