In all fairness, it’s possible last time Thor had to speak English on a frequent basis was in the 16th century, but then he’s spent some time here and he has Updated His Language Pack™ now
The Steve Rogers Language Pack
To be even more fair, Thor 2 happened before Age of Ultron and even in Thor 2 he was speaking in more ‘modern’ language. It is entirely possible that in the four years between Thor 2 and Ragnarok he updated his Language Pack via interacting with the Avengers+other assorted Midgardians on a regular basis