Wargaming had an option for 3rd party websites and developers to get the language that a player has set in-game through the launcher.
This allowed websites (such as XVM and similar others) to show this data, meaning they could compile the number of players by country. On 2/20, they completely disabled this without any notice.
This could be a result of GDPR (General Data Protection Regulation) which was implemented last year, but ultimately only WG knows why.
XVM still partly works when showing player nation as they already have this information in their database.
Thanks to Dakado24 at wotcharts.eu for this information.