how to change default language of SQL server management studio 2008R2?
Using SSMS :

To do so, open SQL Server Management Studio (SSMS) > Right click Server in Object Explorer > Properties > Advanced > Check the ‘Default Language’ property and make sure it is set to the one you want.



using TSQL:

Look up the sys.messages catalog view to check if SQL Server supports a message in your local language.

SELECT msg.language_id, lang.langid, alias 
FROM
sys.messages AS msg
JOIN
syslanguages AS lang
ON lang.msglangid = msg.language_id
GROUP BY  msg.language_id, lang.langid, alias


then once you know the langid use below tsql

EXEC sp_configure "default language", n  -- n = `langid` that you want to set
RECONFIGURE WITH OVERRIDE;


note: you have to restart SQL server and  this is an instance wide setting.