Hi manuxx,
The multilanguage system of Joomla is not something you can really "integrate with" but for which you can build your own multilanguage system. Basically, you have to have your own translation layer (and thus change a lot of things if your component doesn't already handle that from the start).
Joomfish-like systems have the translation layer on their end. So for a developer, integrating with it is really easy. Plus, when we started to work on HikaShop Joomfish was the reference for translations. It was making perfect sense to integrate with it at that time.
Now that HikaShop has been release supporting Joomfish for more than 1 year, continue on that path is easy for us which means that we can give a solution to that rapidly to you.
Supporting multilanguage directly in HikaShop would mean literally months of development. We prefer to spend that time on new features that are requested from a long time instead. We feel that it's a win-win for everyone.