I just finished watching Ratcatcher (1999) and had such a difficult time understanding those Scottish accents. It was so bad that if there were subtitles, the subtitles would need subtitles. The only other film I couldn't understand was Kes (1969), but that film has been infamous for its mediocre sound quality.
I’m not Scottish and do live there….but like anywhere there are a diverse range of local accents, local terms and words for things and dialects, so even a Scottish born person from one area may not totally understand another Scottish person from a different area.
There’s a massive difference between Low and High Scots, Scottish-English for example and then you’ve got Gaelic too!
There isn’t just one or two types of ‘language’ of course used - people from other countries often just think that there’s a ‘Scottish’ accent (which often film makers think too hence viewers complaints at dialects etc used in films featuring Scottish people, especially historical features)….in a similar way that people/film makers portray the English as either posh Londoners or broad ‘Northern’ which is usually a mix of a general Yorkshire and Lancashire accent.
It’s fascinating really.
Near where I am there’s a big proportion of Irish influence and accent too.