How to fix Error 4015 on Windows Server 2025 / Windows 11

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Troubleshooting Guide: SQL Server Error 4015 – Language Not an Official Name

Why This Error Happens

This informational error occurs when a login attempts to connect to SQL Server 2025 using a DEFAULT LANGUAGE setting (either at the login level or specified by the client connection string) that does not correspond to an officially installed language alias in the sys.syslanguages (or sys.server_principals) catalog. This typically happens after a server migration, a case-sensitive mismatch in the language name, or because a client-side application is hardcoding a regional language value (e.g., “British”) that was never installed or aliased on this specific instance. Since SQL Server cannot locate the matching langid, it safely falls back to the server-wide default language (usually us_english) to prevent the login from failing entirely.

Step-by-Step Resolution Guide

Step 1: Identify the Current Server Default Language

  • Open SQL Server Management Studio (SSMS) and connect to your instance via a sysadmin account.
  • Run the following query to check the current server default langid:
  EXEC sp_configure 'default language';
  • Note the run_value; this is the language ID currently being used as the fallback.

Step 2: Audit Official Language Names on the Instance

  • Query the system view to see exactly which names are “official” (aliases) versus simply display names.
  • Run this command to see the valid list of names SQL Server will accept for login configuration:
  SELECT langid, name, alias FROM sys.syslanguages;
  • Compare this list against the specific language string being passed in the connection string or login properties. Pay close attention to regional differences (e.g., “English” vs. “us_english”).

Step 3: Update the Login’s Default Language Property

  • Navigate to the specific login causing the error.
  • In SSMS, right-click the login under Security > Logins > [LoginName] > Properties > General.
  • Update the Default Language dropdown to a valid official name/alias found in Step 2.
  • Alternatively, fix this via T-SQL by executing:
  ALTER LOGIN [YourLoginName] WITH DEFAULT_LANGUAGE = [us_english];

Step 4: Check the Application Connection String

  • If the login itself is correctly configured but the error persists, the issue likely lies in the client application (e.g., a .NET or ODBC driver) explicitly setting SET LANGUAGE upon connection.
  • Locate the web.config, app.config, or connection pooling settings on the Windows Server 2025/Windows 11 client machine.
  • Remove any hardcoded Language= parameters from the connection string unless they exactly match an alias from the sys.syslanguages table.

Alternative or Verification Steps

  • Verify the Fix: Re-attempt the login connection and check the SQL Server Error Log (via SSMS or C:\Program Files\Microsoft SQL Server\MSSQL17.MSSQLSERVER\MSSQL\Log\) to confirm Error 4015 is no longer being raised upon connection.
  • Alternative Fix: If specific regional languages are required but missing, you may need to reinstall the Language Pack component via the SQL Server 2025 setup media to register the missing syslanguages aliases before assigning them to the login.