When I had this error it was because EPiServer cannot tell what culture - region globalization settings it should use.  Can you please post your lang folder contents here or link to them so we can check them.  It's common when migrating from 4.6 to 5.x  sites to have issues with globalization I have found.

Looks like you've migrated right?

Anyway if you read up on localization - you will see there should be a region and a culture component from which EPiServer should be able to find translations for text  I've found that this link gives a good idea of what the /lang folder xml contents are trying to achieve.

http://www.c-sharpcorner.com/uploadfile/ankithakur/globalization_localization_in_dotnet_csharp07032006023510am/globalization_localization_in_dotnet_csharp.aspx

Check if you have any other text translations missing from elsewhere in Edit mode such as [Missing text ...] where it should give a value.  This means that you need to change the code to detect the localization properly using correct 2.0 or 3.5 providers.