By default the reservation form contains a language selection drop-down so users can choose a language. If you already have language selection elsewhere on the webpage, you can remove this drop-down in the reservation templates (in site settings / templates).
You can also pass the lang parameter to the URL containing the checkin24-7 module or the attribute string. The value is a two-letter language code, e.g. EN, DE, FR, IT, ES (upper or lower case works fine). If you don't pass the language parameter, the default language specified in the settings of the checkin24-7 module will be used. Please note that some versions of the checkin24-7 module can detect the current language, whenever the CMS makes it possible for the module to get this information.