TMX cannot be opened due to an error with Invalid culture identifier

 I tried to import a TMX file which was exported from another system called Zanata.

By default, the source language was set to "All" and Trados complained it as an invalid culture code.
I manually changed it to "en", "en-US", and "EN-US" but none of them worked.

I use Studio 2015 and it is updated to the latest version.

 

Could you tell me what code I need to use?