Adding custom language to SQL Server I would like to use SQL Server @@langid variable to return different data based on current connection language (set in connection string or with SET LANGUAGE). This works perfectly well for built-in languages, but there are just 34 of them and I'd like to make sure this technique is extensible.

As far as I understand, languages are stored in syslanguages table. Is there a way to add custom language to running SQL Server instance?
No, you can't add custom languages to the SQL Server system.

An end user won't interact with the system as such: you'd implement multi-language within your database design/application. The system language is irrelevant.