If you have manufacturer product codes that customers can lookup and you have the site in different languages, how does Google know which country version to show?
So if the manufacturer product code is: 44DD7
People in Russia, USA and Spain may all be searching for 44DD7.
But how will google know to show which language, because 44DD7 has no language in the search term?!
Yep. The lang= tag is likely the thing Google looks at. Then secondly, they would try to match your page to the language of the google site the user is using. E.g., google.es users will get the Spanish page.
When your site’s language is not offered at the matching google site, then no results will show.