How Will Going Multilanguage Affect My Seo ?

This is something that bothers me a lot these days. My website is in Bulgarian and it is going just perfect in the results. I want, however, to make an English translation of it.

On one hand, if I make the translation in mydomain.com/en for example, it would be considered simply as a page of my current website, on the other if I make it like a subdomain - en.mydomain.com - this simply looks bad.. I really wouldn’t change to mydomain.com/bg and mydomain.com/en since that would really damage my seo.

I just wonder whether someone has done that and what would be the best course of action…