would there be any difference from a SEO point of view if I would put a language version of a specific site on a subdomain, or on a new domain? so would ru.domain.com be better or www.domain.ru ?
Both sub-domain and a new domain are treated as a separate sites in themselves.. So, I'd suggest go for domain unless there are money issues involved in maintaining a new domain(s).
but would not a sub domain increase the value of the main domain ? So if the sub-domain has a high pr would it not reflect on the main domain ?
I would not completely agree with this statement. Subdomains have their separate existence but still have some relevancy factor with its main domain. Having a separate domain means a complete separate existence whereas subdomain means you are still related to domain! As for suggestion, I would not recommend you to create separate domains for languages, but rather go with subdomains.