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

When Apple announces that they have released it.

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.