Jomres follows the host CMS's language settings, so if you have configured your site to show in English then Jomres will default to that, if it's in French, then the same. You should not need to configure anything in Jomres, everything flows from the CMS.
If you are using Joomla then the official documentation is excellent.
For Wordpress we have found that Polylang is a good choice. If you are creating a multi-language site in Wordpress then you need to ensure that you have created a Page in the format of [jomres:xx-XX] (so [jomres:en-US] or [jomres:fr-FR] for each language).