ASP.NET Core MVC Localization Warning: AcceptLanguageHeaderRequestCultureProvider returned the following unsupported cultures

As documented you can filter logging by specifying the minimum log levels per provider.

In your appsettings.json Logging configuration add Microsoft.AspNetCore.Localization and set the value to Error. In that case the messages will no longer appear in the log.

"Logging": {
  "IncludeScopes": false,
  "LogLevel": {
    "Default": "Debug",
    "System": "Information",
    "Microsoft": "Information",
    "Microsoft.AspNetCore.Localization": "Error"  // <-- Disables the warnings
  }
},

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Update ASP.NET Core 3.0

This issue has been resolved in ASP.NET Core 3.0. As documented here (the official documentation isn't updated yet):

The issue was the localization middleware will log tons if not hundreds of logs if the requested culture is unsupported, image that you will receive 1 warning log per request which is noisy and trouble over the time.

For that we simply decide to change the LogLevel.Warning to LogLevel.Debug to reduce the amount of logs at least.