is there a workaround for the primary domein check?
No. Primary domain check means that the domain mapping plugin associates one unique domain to one blog. Every other domain pointing to the same blog will be redirected to the primary domain. This conflict with Polylang which associates several domains (one per language) to one blog.
I need to use the primary domein check for my other sites in my multi-site installation
In that case, I guess that you would need make a custom plugin to filter the relevant domain mapping option and make an exception for the blog running Polylang.
@sensinum
Please open a new topic not ot pollute this one.