Hi,
Unfortunately there’s no easy way to provide a front-end tool for users to do this (selecting the language they want they content to be added in), if you have user-generated content I don’t recommend mixing multiple languages within the same site because reviews, listings and all the other content will be mixed, and you’d probably to translate everything manually for users and maintain the translation since there’s no way to force users to do this and keep the translation up-to-date. If you add all the content via back-end it’s still possible, but requires code changes for WPML compatibility, please check the suggestions here Integration with WPML - #6 by ihor
For user-generated content (e.g. listings or reviews added by users via the front-end) I highly recommend choosing the primary language and using some automatic translation switcher for other languages (e.g. Google Translate plugin).
Hope this helps.