Why Sql Server management studio use English regional settings instead my local Windows settings?
SSMS is inconsistent. For some data types, it uses comma (I'm Swedish) as decimal separator. For other types, it uses dot.
For money and float SSMS presents comma, but for decimal I get dot. None of them gives me a thousand separator.
SELECT CAST(3000.1 AS money), CAST(3000.1 AS float), CAST(3000.1 AS decimal(9,2))
I guess that one could consider the inconsistency a bug.
SSMS was never designed to be an end-user tool, so we pretty much have to accept what we are given. If we don't like how SSMS present the data returned from SQL Server we can open a feature request for this to Microsoft: https://feedback.azure.com/forums/908035-sql-server
I wouldn't have too much hope for this to be addressed.