Is it possible to edit sys.messages texts? Is it possible to add a new language to SQL Server 2008 and edit SQL error messages with message_id less than 50000?
I don't think it is possible or advisable to edit messages below 50000 in SQL server. You can create user defined messages for ids greater that 50000 using sp_addmessage.

Another possibility is to use RAISERROR to return dynamic messages. The message is returned as a server error message to the calling application or to an associated CATCH block of a TRY…CATCH construct.