I was able to learn 7 languages using different routes. I already knew Russian, I've learned English the usual school + moving to the country route, and I've learned Spanish, French, German, Italian and Latin by mostly reading books and watching online videos and movies.
Having bookstores nearby (especially the Strand) with a lot of interesting books in Spanish, French, German and Italian were key for me. I completely switch 100% of all book reading to the language I'm learning.
I would not be able to learn anything without a steady supply of interesting books.
For Spanish, most movies on netflix and disney will have subtitles and dubs.
For French, there's Arte Karambolage and the newer Disney films are available in french.
For German, there's also Arte Karambolage and newer Disney films and 'Dark' on netflix.
For Italian, there's a great youtube channel by Iliana Zodiaco with interesting book discussions and reviews. Again newer Disney plus films have Italian. And italian is just very easy in itself.
Latin is a lot tougher because there's not as many books in it, and they tend to be not as easy. I've only read Caesar's Gallic wars and Winnie the Pooh. And the grammar is hard. I recommend checking out Lingua Latina Per Se, and youtube channels on Latin Grammar (there's one really good one, 'latintutorial'). Still, this is by far my weakest language. I've started Cicero orations against Catiline, but it's tough going!
I didn't learn to speak in any of these because I don't have daily contact with native speakers, but once I do, it will be fairly easy.