Doesn't it depend on whether they are derived from the standard form or not?
not in italian, except tuscan which is the standard italian, all other are diffrent languages.
to say it is different is to suggest that there is another form of standard language, standard to the group of users. to say it a deviation is to suggest that dialects are not the norms but something inferior to that which is standard. What is standard, howeve, is relative to time and place.
'Differ' seems less judgemental. To the dialect speaker, standard language seems strange. I'd use 'differ'.
The standard language is itself a dialect.
Sometimes a particular dialect can be standardized but after being being standardized it wont be a dialect any more.
Being at variance and out of line with standard form of a language, dialects deviate from it. IMO
One could use either verb to express the idea of this sentence.