Hi @Grzegorz Orda,
Don't think it's possible to download it in Json form and AFAIK there is no API also fetch this list, you may able to take a screenshot of it for future reference.
But you can Get and Update the list of regionalAndLanguage with respect to the user, an open type that represents a user's preferences for languages in various contexts, and for regional locale and formatting that drives the default calendar, and formatting for date and time.
For more information: https://learn.microsoft.com/en-us/graph/api/resources/regionalandlanguagesettings?view=graph-rest-beta
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