In GOTG Drax's language is almost Victorian, yet in GOTG2 his English goes back to normal.
Why might this be?
(Same's the case with Thor in Ragnarok and Thor 1 the contrast is really jarring)
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It's likely due to the fact that he now lives with the other Guardians. It happens in real life all the time. If you spend enough time with a different group of people, you start to sound like them. You see this all the time when people in the United States move to a different place in the United States. They start picking up the common phrases and accent of the area.
In Drax's case, he's now with the Guardians who are much more "relaxed" in their language than perhaps his family before. You mention Thor as well. I thought about this myself after watching Ragnarok. The Thor in the first movie is much different than the Thor in Ragnarok. It's probable that we have the same case here. Thor has been spending a lot of time with the Avengers, and consequently have picked up some of their mannerisms, like the sarcasm, humor, etc.
Another notable case is with Wanda. In Age of Ultron, she has a thick accent because she is from Sokovia. Throughout Civil War and eventually in Infinity War, the accent is basically gone. This is most likely due to the same thing. She's been spending a few years in New York with natural English speakers and has picked up the accent. (Although, I think I read one time that she purposefully spoke differently after Civil War because she was on the run, but either way, it's an explanation.)
In short, people's language and accents can change when they change environments or just spend enough time with a certain group of people.