There is no way to enable captions by default when folders are shared. For example, my girlfriend is hearing impaired I shared my library with her but for every video you must enable the captioning, where the main account has captions with the default language set and working perfectly. If it means anything, I am a Plex Plus subscriber.
You might want to check out her account’s language preferences:
While signed into her account, open Settings > Account and edit the AUDIO & SUBTITLE SETTINGS:
Check Automatically select audio and subtitle tracks
Set her preferred audio/subtitle languages
Set AUTO-SELECT SUBTITLE MODE to Always enabled
Keep in mind that auto-selection won’t modify any previous manual selection of audio/subtitles.