Multilingual website proper url, how should it look like?

Tell me which option is a proper option and if any of these options has some serious flaw that one should avoid at all costs

English website root folder installed main websie www.mainsite.com

subsite english version www.mainsite.com/help

subsite german version www.mainsite.com/de/hilfe

PLEASE note that help and hilfe are not just pages but entire websites created with wordpress subsite. IN other words those are separate websites/blogs and not just pages of the mainsite.

would this be okay from the SEO point of view? Basically the german version of the website having /de/ in its url? English version not having en.

But version being subsites.

mainsite simply being in english. Would that be a big problem?