Joomfish isn't used for translation in Jomres. That's because Jomres language handling pre-dates Joomfish's language handling, and because Jomres language handling is more appropriate to Jomres both in how properties are managed, and considering that Jomres is usable outside of Joomla.
That said, it's worth noting that Jomres can work alongside Joomfish in that it will detect when Joomfish has changed the current language and ensure that it uses the appropriate language file for the change.
Jomres uses long short-codes such as en-GB, fr-Fr to define languages. This allows us to have different languages for different regions. For example, in British English we use the word Tariffs, whereas our American cousins prefer the word "Rates". Having an alternative en-US long shortcode we can make that distinction in seperate languages. Joomfish on the other hand uses short short-codes, eg en, fr in it's default configuration. This can be changed in Joomfish's Language Configuration option but it doesn't need to be changed as Jomres will detect when the variable 'lang' is set to a short-code, and if a corresponding long code is found, then the appropriate language file will be used.
If you are using Joomla 2.5 you may be having problems with using Joomla's language functionality. If you simply use Joomla to host your Jomres site, you can install and enable the jomres_language_plugin_bot plugin, which will enable the Jomres language switcher to switch the Joomla language to a new language (if it's installed).
This document is copyright Vince Wooll/Woollyinwales IT, 2011. All rights reserved.