How to fix Error 4019 on Windows Server 2025 / Windows 11

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Troubleshooting Guide: SQL Server Error 4019 — Invalid Login Language

Why This Error Happens

This error occurs when a login (either in the connection string, application configuration, or user default settings) specifies a LANGUAGE value that does not match any entry in SQL Server’s sys.syslanguages system table. This commonly happens after a SQL Server upgrade/migration (such as moving to SQL Server 2025) where the Language Resource files were not fully installed, or when a default language was manually set via ALTER LOGIN using a typo or deprecated language name (e.g., using regional variants that were deprecated in newer builds). It can also occur if the server’s default language setting was corrupted during a collation change or master database restore from an incompatible version.

Step-by-Step Resolution Guide

Step 1: Identify the Invalid Language String

  • Open SQL Server Management Studio (SSMS) and connect using a Windows Admin account (since the SQL login itself may be broken).
  • Run the following query to see all valid languages recognized by the instance: 
  SELECT * FROM sys.syslanguages;
  • Compare the output against the language name referenced in the error message (usually found in the SQL Server Error Log at C:\Program Files\Microsoft SQL Server\MSSQL17.MSSQLSERVER\MSSQL\Log\ERRORLOG).

Step 2: Correct the Login’s Default Language

  • Identify the broken login using: 
  SELECT name, language FROM sys.sql_logins;
  • Reset the specific login to the server’s default language (usually us_english) using the ALTER LOGIN command:
  ALTER LOGIN [YourLoginName] WITH DEFAULT_LANGUAGE = [us_english];
  • If the application connection string is forcing a specific language via the Language keyword, update the connection string on the client machine or IIS configuration to remove the invalid parameter.

Step 3: Repair Missing Language Resources (Server-Side)

  • If sys.syslanguages is missing standard languages (indicating a corrupted installation), launch the SQL Server 2025 Installation Center from the original setup media.
  • Select Maintenance > Repair and choose the specific instance name on your Windows Server 2025 machine.
  • Allow the repair utility to restore the default language resource files located in the Binn directory, then restart the SQL Server (MSSQLSERVER) service via services.msc.

Step 4: Restart Services and Clear Cache

  • Restart the SQL Server service to ensure the language cache is reloaded into memory.
  • Open PowerShell as Administrator and run: 
  Restart-Service -Name 'MSSQLSERVER' -Force
  • Attempt the login again using the previously failing credentials to confirm the queue has cleared.

Alternative or Verification Steps

  • Verification: Attempt to log in via SSMS using the specific login that previously failed; if successful, check the sys.dm_exec_sessions view to confirm the language column now displays a valid, officially supported name.
  • Alternative: If the repair fails to restore language files, manually drop and recreate the specific language using sp_addlanguage (though this is deprecated) or simply force all client applications to omit the Language parameter entirely, allowing SQL Server to default to the server-level setting.