I've made some changes to the plugin which fixes some issues with getting multilanguage URLs to work in the virtuemart plugin.
In plugins/jamegafilter/virtuemart/helper.php in the method getFilterItems($catid) inside the foreach, first line there:
vmLanguage::setLanguageByTag($this->getLanguageCode($lang));
This sets the correct language for the virtuemart router. However! The first time the JRoute:🙁) method is executed for the virtuemart router, the language is reset back to English... this will cause issues if it starts to index urls for another language. To fix this, simply add JRoute::('index.php?option=comvirtuemart'); above the foreach loop. This will trigger and create the route instance once and then you can freely change language inside the foreach loop.
Apart from this, we've also added the translation method in Joomla to the customfield code in the plugin, so we can use translation strings for the customfields.
Considering multilanguage support is builtin in the plugin, we thought it would "just work" but apparently it was half done. The code above fixes pretty much everything.
Edit: I forgot to mention, we added the getLanguageCode method too, which simply just changes the $lang variable into a language tag string.