A recent ranking rules change (t/169960) states diff names must be understandable by an average human player. However, osu is a language-neutral app and not all player understand English. If things like "easy", "normal", "hard", "insane" or even "hyper", "another", "extra" is translated according to user's preference, it would be good.
(Also, mappers should be able to use non-English custom diff names, provided that an English/romanized version is present, like song name)
(Also, mappers should be able to use non-English custom diff names, provided that an English/romanized version is present, like song name)