Best SEO solution for bilingual site

I have a xyz.pl website. The user can switch to the second language version on it, then he will go to xyz.pl/en.

Is this a good solution for seo? If so, set noindex to pages in the second language?

Yes that is good because those /en pages can rank as well

First you have to ask yourself if you want to rank for the foreign language keywords as well.

If so, then you need separate URLs for each language...
- language directories like /en/ or /fr/
- you need to index all of them
- use hreflang in the correct way
- translate the url because it would push the CTR on SERPs.

example.com/en/how-to-buy-a-new-bike
example.com/fr/comment-acheter-un-nouveau-vélo

If not, you do not need to change the URL. Then you should just create /en/ to keep the possibility open for later.