Personal opinion, but to me accents, dialects, or regional and/or dated slang are like seasoning for a meal. Throwing in something here or there can add flavoring. If you go overboard, the entire thing tastes like a mess.

I haven't seen Brave so I can't get too specific about how I'd personally transliterate that, but I guess I'd caution against liberally dropping 's and ach/ochs and shit like that to the point it's a borderline-to-not-so-borderline offensive take on Scottish accents. But a little bit here and there can add to your writing. Key word being little.