We have different domains for each language
And then we have different sitemap.xml for each
I'm using the following solution in .htaccess after all domain redirects and www to non-www redirection.
# Rewrite URL for robots.txt
RewriteRule ^robots\.txt$ robots/%{HTTP_HOST}.txt [L]
Create a new directory in your root called robots. Create a text file filled with the specific robots information for every domain.