How to determine the language of a user visiting the site for the first time?

The language of a user visiting the site for the first time is usually determined by the Accept-Language HTTP header that the browser sends automatically.

In ASP.NET Core, for example, you can retrieve this header from the request object:

var userLanguages = HttpContext.Request.Headers["Accept-Language"].ToString();
// Usually, this is a string like "ru-RU,ru;q=0.9,en-US;q=0.8,en;q=0.7"

// You can take the first language:
var firstLanguage = userLanguages.Split(',').FirstOrDefault()?.Split(';').FirstOrDefault();

This method allows you to determine the user's preferred language without using cookies or sessions. If more precise detection is needed, geolocation by IP can be used, but it is less reliable and requires additional services.