Symptoms

I tried to download Language Pack from K2 partner and customer portal. But I can't access it due restrictions in my account. The error I see when loading the K2 Designer is: Could not load file or assembly 'K2Field.K2Forms.Controls.SetLanguage' or one of its dependencies. The system cannot find the file specified.
 

Diagnoses
This is due the Language pack from the Community was used on some forms. This is not supported by K2.
 

Resolution
Copying the DLL from the Extracted package.

That was downloaded from 

multilingual_1.9K2Field.K2Forms.Controls.SetLanguageK2Field.K2Forms.Controls.SetLanguageControlsinDebug

Copy the K2Field.K2Forms.Controls.SetLanguage.dll

To the : oInstall dir]:Program Files(x86)]K2 BlackpearlSmartforms Designerin and rInstall dir]:Program Files(x86)]K2 BlackpearlSmartforms Runtimein folders and then Perform an IISRESET.

Resolved the issue. K2 doesn't support that package. Our recommendation would be to replace the Language control with our control package.