How many messages there will be depends on how many people take advantage of that option.
There are no restrictions on language, AFAIK (there may be restrictions on what characters are supported, which could indirectly limit the language). All messages will have to be approved, to make sure there are no ads or messages like 'XYZ sucks/rulze', etc, so someone at Larian will have to understand it or be able to get it translated.
There will be a kickstarter update (at least) when the development gets close to the point when Larian will need to start organizing all the feedback in order to get it into the game. I'm not sure how far in advance of when the game will be finalized that the kickstarter survey will go out to everyone (to confirm addresses for physical tiers or add-ons, etc), but I would assume that would be the last chance to submit anything for the tiers that require feedback.
The maximum length has not been determined yet. It may depend on how many people write messages, and what the average length is (so if fewer people than expected write messages, or there are lots of short messages, maybe a few longer ones would be ok).