site language
 

Is it a good practice to offer translated versions of site menu, niches etc. with subdomain but keep promo material titles and descriptions in English from seo and user experience perspective?

Not good to keep them in En on subdomains, should translate them

Spanish, Italian, French, German, maybe Portuguese...these are the top languages to translate.

Decide what language demographic you could or are attracting now. No shortcuts, translate everything on and off site into that second, third, fourth language.