Hi, it really depends on the style of manga you're translating and what you're looking for. I like to use the same font family, just leaving the thoughts in a light gray color and italics.
Some fonts that can be used and I recommend for both, normal speech bubbles and monologues: SS Pretzel Family, Boba Family, Blah Blah Blah Family, CC Marian Churchland, Tight Spot BB Family, Digistrip Family, CC Face Front and CC Ask For Mercy Family. All can be found for free online.