I will let the mods judge about the relevance of this post and I will let the readers judge about the usefulness of its content. (long post incoming!)
I am an experienced language learner and, according to general definition, a polyglot (I know five languages at an advanced level). Throughout the past six years, I have studied (which absolutely doesn't imply I know) over 20 languages. Some I just tested for a challenge, others for the fun of it, others due to a specific goal, others for passion, and so on.
Until last year, I was the regular guy who could very well have posted countless times about "which language to learn between x, y, and z?". I faced this dilemma between Slavic languages, then Turkic languages, then Uralic languages, and on, and on, and on. I realized, though, that whilst other people's perspective is most welcome and useful, you still are the one who knows yourself best and who will take the final decision.
My post has no pretention to replace the remarkable contributions of this subreddit's members. I just want to provide my own contribution in a slightly different way and I truly hope it will be of use.
Language learning, in my opinion, should be taken as it is supposed to be: a passion, a hobby. Just like reading or exercising. I love comparing it to sports: instead of training the body, we are training our brain here. But like all things, we tend to make progress when we actually love what we do. I know it sounds logical. My point, however, is this: when choosing a language, it is more than just a set of sounds you appreciate, quirky grammar, and fascinating words. These aspects are marvellous but they're just a part of the discovery. A language is not only a communication tool used by a group of humans to convey their thoughts, it also is the soul of a culture, the soul of a people, it has a deep history. A language is as complex as the human is. And this is what's magical, in my opinion.
When choosing a language, you basically make a choice that might affect your life in a way you never could imagine.
Each language I've studied, I can associate various people, songs, phrases, images, all kinds of memories. Learning or knowing x language impacted my personality, my way of thinking, even my routine to some extent. It opened doors to worlds I never knew of. It offered me one of the best gifts of my life: more than mere knowledge, but humanity itself.
"If you speak to a man in a language he understands, that goes to his head. If you speak to him in his language, that goes to his heart." Nelson Mandela
This is the quote that got me started and I think it is a beautiful motivation.
Talking about motivation, it is (I guess) the very first factor you need for choosing a language to learn. That said, mere motivation is not sufficient. Then you will need passion (that inner fire you have for the language) and discipline (building on motivation to make the language a part of your daily life). Unless you're a dabbler (which is fine), you want to actually stick with the language you choose in order to reach specific goals (perhaps fluency). Obviously, you can't do that by clicking your fingers. It takes plenty of time and effort. The good news is you can optimize that.
Many of us tend to suffer of "language wanderlust", deeply related to "language infatuation". That is you hear x language and boom you get infatuated with it. Or you read about it and you're struck with fascination. Or perhaps you fall in love with a native speaker whose language then begins to attract you (personal experience can attest it lol). This feeling is good but insufficient to make a choice, as you might guess. You need to dig. It's a bit like human relationships. You won't choose someone for the first impression but more for who they are inside and how they make you feel.
How does the language make you feel? Why are you attracted to it? Ask yourself, evaluate this, for each of your language candidates.
Besides, important questions: Are you attracted to/do you feel connected to your language's culture? Are you interested in its native speakers and their way of thinking? Are you interested in your language's literature and cinema? Are you interested in its related traditions and heritage?
You also need to be realistic, though not overly. Some of us can do two or even three languages at once but the general rule of thumb is focusing on one language until you reach B1/B2, before adding on the next one. You need to remember this: a language, just like a sport, needs regularity. Every day, in our case. It is crucial. Even just 30 minutes a day can do wonders.
How much time can you dedicate to your target language(s) per day? How can you optimize your time? How can you make the language a part of your daily life?
Now, some general fears that often create hesitations. That pertains to "difficulty". I dislike metrics like "easy" and "difficult" because it's too subjective. I was told countless times Finnish is terribly hard, yet it happened to be one of the easiest languages I've studied. I was told Spanish is very easy, it turned out to be a nightmare to me. Instead, I tend to analyze languages through "differences".
How different is your target language from those you know? Research about its phonetics, syntax, general grammar, quirks. How confident do you feel to tackle potential differences (say SOV word order instead of a potentially familiar SVO)?
By the way, mindset is of utmost importance. If you think x language is hard, then it will be. Put those useless metrics aside. If you answered positively to the previous questions, then "difficulty" should be at the bottom of your concerns. Your mindset might rather look like this: This language is an opportunity to teach me new things about how humans think, how language and culture are deeply intertwined, and how much I might change by studying it. Any differences (word order, subjunctive, tenses, cases, etc) can be easily tackled through exposure and practice, anyway.
Also, get out of your native language. Stop comparing. Even if it's similar to your target language. Each language is unique, else it wouldn't exist or it would just be a dialect of one language. Even Danish and Norwegian, which are extremely close, have their share of specific quirks. And that's okay. This is the richness of human languages. Try to adopt the logic of your target language. Observe. Analyze. Mimic. Practice.
Personal experience: Russian seemed very weird to me. Verbal aspects and syntax specifically left me with many questions at the beginning. I eventually answered them and solved the problem when I began to think the Russian way, adopt the Russian logic. It worked out in the end and now it all makes a lot of sense. Comparing to my German or Italian just added more confusion, I let go of this useless comparison and instead fully embraced Russian for what it is.
Are you interested in the specific logic of your target language? Do you see yourself trying to adopt it?
Now, another important aspect which isn't easy to tackle. What's the future of the "relationship" between you and your native language? What are your short-term and long-term goals? Do you want to know the language passively or actively as well? Do you want to read literature and/or just be able to talk about general topics? Do you want to travel to the native country of your target language eventually?
One general tip as well. If you're hesitating between languages of the same group, be very careful. You face the danger of mixing them up and it's not a beautiful experience. Pick one, achieve your goals, only then can you add on another one. How do you choose the first happy candidate? Go through all of the afore-mentioned questions and listen to your gut feeling, your heart.
Listen to the sound of the candidate languages. Which one stands out the most?
Finally, on practical aspects. They shouldn't impact your final decision too much but they obviously can play a significant role.
How accessible are resources in your target language both online and offline? Do you know any native speakers? Can you use the language where you live? Do you know online communities (Discord servers, subreddits, etc) where you can find native speakers and extra resources?
Bonus tip for resources: look for "Easy Languages" on YouTube. They have channels/free content for a dozen languages. Excellent quality. Amongst the languages (aside from the most famous ones like Russian and Spanish) are Catalan, Welsh, some Arabic dialects, Kurdish, Turkish, Swedish, and many others. Also, generally speaking, be a searcher! The Internet has so many resources which you don't know of. They're waiting for you!
I hope this very messy post (I'm very tired as I'm writing this and the decision was completely spontaneous) will be useful. I might edit it to add extra thoughts or recommendations and I'm absolutely open to any useful criticism, personal perspectives on the topic, etc. Fellow language learners, I wish you the very best in your endeavours! :)
EDIT: grammar.