Starting with v.2020.1, each user can have a different Default Culture.
The Default Culture sets the language and culture, which the user prefers for User Interface, Notifications, etc.
The culture is a string, identifying language, sub-language, country and culture in a hierarchical way. You can read more here:
"en-US" - this is the culture string for English in United States.
You can see the list of all supported cultures here:
NOTE: If a notification or a UI element do not support the requested culture of the user, the closest possible culture is selected. For example, if the user requests culture en-WS (English in Samoa), but this is not available, non-specific English message might be used.
In some cases, Automatic Transliteration (transcribing of characters between Cyrillic, Latin, etc.) might kick-in and provide better message. However, Automatic Transliteration is available only for certain base cultures. If automatic transliteration is not available between the requested and available culture, the closest possible culture is used.