Basically, the option to search for parties on other block which is accessed by selecting a quest then picking the option with just the name only shows players whose language setting is visible as JP in party view. In fact each member of the party has their language set to JP when looking through search results. It's not merely wrongly displaying party members' localization either, as I couldn't find my teammate's party with language set to EN, while the ones with language set to JP were clearly visible. While that option has been somewhat depreciated with the advent of multiblock joining, I have no idea whether multiblock matching is also affected by it, and it's still a nice option to check whether somebody is running Ult MPAs without launching the quest.