keyboard language in watch

It's not possible yet.

https://www.apple.com/watchos/feature-availability/#quicktype-language

To ask Apple for that, use

http://www.apple.com/feedback

Alexandr0212 wrote:

and when can I do it?

Nobody in these forums has any info about Apple's plans. Use the link you were already given to tell them you want it.

When Apple announces that they have released it.