It is interesting that Japanese, Russian and Thai benenefit more (30 %) from brotli, than latin languages (25 %). This is because of the utf-8 context modeling in brotli.
[1] https://github.com/google/brotli/blob/master/dec/context.h [2] https://tools.ietf.org/html/rfc7932#section-2 [3] https://tools.ietf.org/html/rfc7932#section-7
Though for languages like Korean and Chinese (whose size is more inline with latin languages) we see 27.5% improvement, which is most likely due to context modeling.
Therefore I assume ratio improvement is split ~50/50 between these two. It was easy to verify that by compressing data with `brotli --window 15` and comparing ratios there, but I was lazy there. I'm sorry.
PS. I've also skipped NFC/NFD part of the post which is very interesting for Korean, where NFC normalized text occupies 30% less space. It also gives additional ratio 5% for brotli and 15% for gzip.
What was saying is that there is a strong correlation between size of the data I was compressing and compression ratio improvements over gzip.