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Go to the Messaging tab, Languages page and make sure your desired languages are specified there, and a default language is set.
Then go to the Messaging Translations tab and choose "Download Messaging Translations"
In the Excel file, you can translate any messages into your desired language. Note: CommCareHQ doesn't support accents in these messages so they will need to be removed from any text. For French users, the Messaging translations are here: translations.xlsx
Now the messages will get sent in users desired language.