A commision of inquiry is NOT similar to a Grand Jury or congressional hearing. To the best of my knowledge its terms of reference seldom, if ever, encompass non-citizens who reside in other countries.. But as others pointed out, it can summons leaders who are citizens within that country.
BTW, I had understood that JWs refused to swear on the Bible because taking an oath is considered unscriptural and instead, like non Christians, unbelievers, athiests, etc, read out a statement that they would answer truthfully, aware of the legal sanctions if they did not tell the truth. The days of being legally forced to swear on the Bible are well and truly over.