I think the problem is that you use a Region code, so can you add english language in your app(only add it in backend, no file needed), make it default then remove the es_MX language from backend, then rename your translation folder from es_MX into simply es and zip it and upload it into mailwizz and make it default.
Another thing, if you take a look inside /apps/common/framework/i18n/data/es.php or /apps/common/framework/i18n/data/es_mx.php files you will see how exactly formatting is done and if needed be, you can alter those files a bit.