It's definitely for faster queues. People have been doing it in Dota for years.
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Nothing in Duty Finder requires you to write a thesis in order to communicate. You say {Hello}, {This Way}, and {Good Game!}. If other languages in a DF party "depresses" you, then you need to work on being less xenophobic.
You are not getting my point. I ONLY tick english because I don't want to read silly french or german in duty finder, I want to play with english speaking players. Yet I always get french people and sometimes german players that don't know a single word of english.
In dota you can report them for doing so though. I don't think we are allowed to kick players who don't speak the language they searched for in this game.
What makes you say there's a "language rule"? The only description I see for the language option is its mouse over text that says "Parties will be formed with players who have selected the same languages." It gives no criteria for what reasons you should select one. As others have said, it can be great fun to party with someone from a different language/culture, particularly when we're provided with the translator tool.
This is going to be a great read after I get out from work...