What language do you think is the easiest and quickest to learn? As an English speaker I would think the easiest one is among French, Spanish or Italian. Just wondering :)
Motivation will make a huge difference in whether you keep going with a language or not. For me, any language I want to learn will be very difficult. So, I work on languages that offer me motivation.
1. Will learning this language give me access to or improve something in my life? (Family, Friendships, job opportunities, hobbies, travel etc.)? 2. Is there any regular opportunities to enjoyably use the language? (Friends, books, music, movies, video games, podcasts, etc.)
3. Am I attracted to something about the language for it's own sake? (the look, the sound, the script, the feel of expressing it yourself, etc.)
4. Do I want to learn this language?
5. Do I have to learn this language? 6. What is a realistic goal for my time and energy? (Fluency? Non-fluent but easily conversational? Able to make basic conversation? etc.)
Your answers to those questions should give you a clue as to which language you might want to start with. If not, the good thing about Duolingo, as you've discovered, is that you can experiment with more than one language to get a feel for which one(s) you might want to pursue. :)
As examples, here are the three languages I spend the most time and energy on:
ASL (American Sign Language) makes it easier to communicate with some of my friends. One of those friends I hang out with each week providing me regular practice with a fluent signer. While I don't think I will reach fluency, I want to be moderately conversational in ASL. Of the languages I'm learning, I'm going to devote the most time and energy to this language.
Meanwhile, I'm studying Japanese for fun. It is very different from English. The the sound and script are beautiful. I don't watch much anime these days, but if I do it will be cool to follow along the original language. Also, there is just a coolness factor to Japanese. I'm not attempting fluency with this language because I don't have the amount of time or energy it would require. But, I would like to know some basic vocabulary and grammar, be fluent in 2 of the 3 scripts, and learn some kanji. I feel like these are reachable goals so I'll work towards them.
Last but not least there's Spanish. Already, I've used it to help me locate a friend who ended up in the hospital in a non-English speaking territory. I also helped someone find the right bus to get them home, and I've ordered food at a restaurant. Sadly, my friend and conversation partner moved away. So, I don't have regular opportunity to use it. If I enjoyed novellas, I would put more time and energy into studying Spanish. Since I'm also not planning to be fluent in Spanish, again because of time and energy, I would like to be able to quickly develop basic conversational skills if necessary or my friend moves back. So, for me a realistic goal is to go back through Duolingo's course now and then.
It uses some features of certain other languages, so if you know Esperanto it might be easier to learn those. Also, by having already learned one secondary language, it might be easier to learn more, even those unrelated, by having gone through the process once before and knowing what it'll take with regards to time investment, methods that work for you, etc.
Or something like that.
EDIT: Kinda like what OldPalHappy said below lol