Is it just a coincidence that the farther out from the UK you go the more weeks it takes to learn the language?
http://multilingualbooks.com/languagedifficulty.html
i.e. category 4 on the original web page.In fact, if you check the map, there's a gray spot on the north of Spain/South of France. That's Euskadi (Basque Country area).
[1] https://www.reddit.com/r/MapPorn/comments/7dith2/language_di...
They are also learning full time at one of the best language schools in the world.
The average person taking average classes part time wouldn't learn nearly as quickly as they do.
And indo-european was a pretty complex language, with all the cases, declensions, plural modes, etc. The more of that is lost, the easier it is to learn.
The more conquests, trade and migration there was, the simpler the language is. That until you consider vocabulary.