Not really a Mendix question but more a react question. There are some blogs about this subject like this one: https://phrase.com/blog/posts/localizing-react-apps-with-i18next/ or this one: https://www.brihaspatitech.com/blog/how-to-localize-react-js-web-app/
Regards,
Ronald
The current Mendix session locale is available at:
mx.session.sessionData.locale.code
Here on the forum that code returns ‘en_US’ for me.