Music historians of reddit: why is it that European metal bands like At the Gates and Amon Amarth perform their vocals in English, despite it not being their first language? What's the history behind this subgenre?

It really feels weird that a question about Sweden's melodic death metal scene meets the 20-year rule (I am 22 and it makes me feel old).

I have a follow-up question: can the increase in frequency of non-English stuff in the mid-1990's, particularly in folk metal (which seems to have exploded starting in 1997, so just barely meeting the 20-year rule) and the black metal band Enslaved, be traced to nationalist tendencies in extreme metal, such as Varg Vikernes's influence on the Norwegian black metal scene?