I'm an English-only speaker who has at times tried to learn other languages (Spanish, Mandarin, Italian), but aside from some very basic proficiency (I hesitate to even call it "proficiency") in Spanish, I never got anywhere useful in anything else.
Seeing all the comments here makes me feel a bit bummed out. I still would really like to become proficient in another language one day, but it seems like it's really a "use it or lose it" proposition, and I don't think there's any language I could learn that I'd have the opportunity to use often enough to retain it, assuming I could immerse myself enough to learn it in the first place.
(Well, I suppose I could learn my wife's native language, but I'm a little skeptical that she would want to talk with me in it often enough to to keep it fresh in my mind.)
Speaking is use it or lose it[1] but understanding stays. If you spend on average twenty minutes a day listening to content that is at a level where you understand most of it for twenty years at the end of it you will have no trouble understanding that language in most contexts. You can even have year long gaps as long as the average works out. Learning to understand a language isn't hard (if there's sufficient learner content) it just takes a lot of time. Of course if you have enough free time you can do 4 hours a day and get there in less than two years. Once you are at that level you are in an excellent position to learn to speak the language even if you have to repeat that part of the process if you leave long gaps.
If you want to give Spanish another go I can recommend dreamingspanish as they have just a ton of excellent learner content on various levels.
[1] to the point that I sometimes have trouble expressing certain concepts in my native German even though I live in Germany because so much of my life happens in other languages.
You can go in deeply enough that you won't forget it even if you pick it up later in your life, but the trouble is you cannot know whether (or not yet) you have reached that level. If you were to pursue that as a personal project, spending a few months or one full year abroad to fully immerse and then following up back home with some routine of combined reading of books in the language and occasional meetup group to have some occasional practice/use is what I would recommend.
BTW, Spanish and Italian are similar enough that there could be confusion in the learner's brain (happened to me - after two years of Spanish in school, self study of Italian from six audio tapes failed because for each word, the Spanish version was promptly recalled instead of the newly acquired Italian one).
IMHO, it's worth it, and from what I read in the UK interest in foreign languages is declining (and it corresponds with personal experience, when I studied a bit of French and Russian, the French course had no Brits in it, and the Russian course just one, everyone else was a foreigner).
As a linguist I may be biased, of course, but I would encourage you to pursue at least one language other than English more deeply (instead of, say, dabbling in three superficially), because it opens up a new horizon being able to navigate a culture without translator and reading its literature in the original. There are certain words, phrases and sayings in each language that when you "get" them you feel like "I no other language could one say this better!", whether it's Danish, English, Spanish, German or Latin.
One of my Ph.D. advisers was British and the other one U.S.-American, and I won't forget
PS: Is there a link without paywall to the original article?
Yeah, it can be hard, if the second language is not English. It easy to find places to practice English and get some other benefits as well. (Like NH for example, where you can read news or talk with people)
I'm learning German, I finished the Duolingo course, and now I'm just reading in German. Books, news sites, and suchlike. It is not the best way, I know from my experience of English: if you don't speak in language, you cannot speak it; if you don't listen it you cannot hear it. My plan is to let it go, as it goes, collect a big vocabulary and the feel for the language, and then maybe take some courses, to polish theory and get an experience of writing, talking, or otherwise generating German sentences. I learned English in this way.
Seeing all the comments here makes me feel a bit bummed out. I still would really like to become proficient in another language one day, but it seems like it's really a "use it or lose it" proposition, and I don't think there's any language I could learn that I'd have the opportunity to use often enough to retain it, assuming I could immerse myself enough to learn it in the first place.
(Well, I suppose I could learn my wife's native language, but I'm a little skeptical that she would want to talk with me in it often enough to to keep it fresh in my mind.)