Immersion....
We've been very fortunate to live in 3 non-english native countries/regions.
Belgium (3 official languages (Flemish, French and German) and most folks in Brussels speak English too). We choose to live in the Wallonian area as it was central to travel to other parts of Europe (see WWI / WWII German invasion paths) and very few people spoke English, mostly French so one learned French conversational/daily quickly. Kids hated it, but they were early teenagers BUT we learned a lot without falling back to English.
Then we moved closer to Utrecht, NL, sort of picked up a smitten of Dutch but most Dutch, like 80% speak English. So it was difficult to not speak English.
Off to Japan, met some of the best English speaking folks there in some of the most remote areas (like where the snow monkeys live). But when we were on our own (no interpreter) we had to pick up not only parts of the language but also the three written forms, Kanji, Hiragana, and Katakami.
I studied Spanish (4 years) and German (2)... I have to use Spanish a bit when I return to work the vineyard but my German has faded away, I think I could read a simple menu now.. Bitter, eins Bratwurst und zwie bier.
We traveled to a lot of other countries, the problem is most folks wanted to practice either English on us not the local language. So having spent the time in SE Belgium where a lot of the locals didn't want to speak English is where I picked up the language the quickest. My wife OTOH picked up Japanese writing and language quicker (but she's dyslexic) .. go figure.
Immersion IMHO is the best way along with study/flashcards of the conversational language.