The code in question only fallback to netural version.  

https://world.optimizely.com/blogs/Mark-Hall/Dates/2020/4/fallback-languages-for-catalog-content/

Aslo full support is available in commerce 14, you should consider upgrading

Thank you for your reply. In my previous message I forgot to add that I've modified the GetNeuturalLanguage method. It is my current definiton

private static string[] GetNeuturalLanguage() {

               return new string[] { "en-GB" };

}

I've tested the fallback language in Foundation project (.NETCore version) but unfortunetly it doesn't work.

These are my steps to configure the fallback language in the Foundation project.

1. Go to CMS -> Admin -> Config and I enabled en-US language
2. Go to CMS -> Home page -> Language Settings and I set fallback language as below
   

      3. Go to Commerce -> Fashion catalogue and I ticked the English (United States) checkbox and published the catalogue.
               

      4. Go to the pages

https://localhost:5001/en/fashion/womens/womens-tees/ - it works ok

https://localhost:5001/en-us/fashion/womens/womens-tees/ - I got a 404 error

Do you have any idea what I have done wrong?

https://world.optimizely.com/documentation/Release-Notes/ReleaseNote/?releaseNoteId=COM-12848

You set the efallback language on the catalog level

Thanks. It works well in the Foundation project (.NET Core version). 

But in the .NET Framework I still have a problem.
I've added this code to the Foundation project (branch full-framework/master) and I've noticed that this implementation does not work for this product
http://localhost:53396/en/fashion/womens/womens-shoes/p-42708712/
http://localhost:53396/fr/fashion/womens/womens-shoes/p-42708712/

This looks like the same as the issue in my application. Do you have any idea why the above example doesn't work?