The first thing you need to do is consider your goals and motivations. Those factors that are most important to you are going to be the things that ensure you retain the long-term motivation required to learn a language. People choose a language for a multitude of reasons, here are some you might consider:
Personal interest—Personal interest means you find the language inherently interesting and want to learn it for the joy of engaging with it. Some people learn languages because they think the grammar is cool, the language sounds beautiful, or they may simply like the culture, food, or music. Personal interest is generally a very powerful motivation that can persist for a lifetime.
Work—Knowing a language can create job opportunities and improve the look of a CV. Generally, the languages chosen are widely spoken or are the language of a country that your own frequently trades with. While it’s true more jobs benefit from fluency in Chinese, French or Spanish, other languages are still useful in a globalised world.
Utility—Utility means how useful the language will be to you personally. The languages with the most utility are typically those that are spoken where you live. Utility also comes from learning the languages of places where you would like to live or visit. Because many learners learn to communicate, utility is often a very motivating factor.
Practicality—This means availability of resources. If your target language is relatively obscure, resources in your native language may not be easily accessible. Difficulty finding resources or interesting content can severely hurt your motivation and interest, so learners who learn such languages are typically more motivated by other factors. Having family or close friends around that speak the language can mitigate the resource problem. Learners of more uncommon languages often use another more popular language as a bridge. For example, if you would like to learn Catalan, it will be helpful to learn Spanish first so you can use more resources.
Family—This means relatives you wish to communicate with better or a family heritage language. If you want to use members of your family to practise with, it is a good idea to make sure they are willing to help first. Talking with a complete beginner is rarely an interesting task for the native speaker, and as a learner you will only become engaging to talk to once you are at least at an intermediate level.
Ease—Languages that are more similar to ones you already know are significantly faster to learn. If you want to get to a communicative level faster, learning a similar language is better. If your only language is English, then the fastest languages to learn are Spanish, Swedish, Norwegian, French, Dutch, Portuguese, and Italian. If the target language uses many different sounds, has little common vocabulary, or has a radically different grammar, you must be willing to invest more time into it. For a quick idea, check the FSI ranking, which serves as a rough approximation for someone who is dedicated but speaks only English. You can see that the hardest languages take over three times as long to reach a certain level as the easiest ones. While ease matters, in practice, people learning “harder” languages out of personal interest tend to be more successful than those who merely want to learn a language and simply pick the easiest. This is because the former has more motivation. Rather than thinking of difficulty, it is better to think in terms of the number of hours with the language it might take to reach a certain level. This is because languages you are bored by will be hard for you to stick with. You won’t enjoy the necessary hours of exposure. On the other hand, learning a language you love can be a lot of fun, even if it takes a bit longer.
The most important of these six categories is probably that of personal interest. Whatever language you really want to learn the most is the one you should probably choose. At the end of the day, you choose your own life priorities. What you prioritise needs to extend from what makes you happy. While the other five criteria can make you happy, they only do so indirectly. The consequences of learning an easy language or a language useful for your career are what make you happy. For languages you rank highly in personal interest, engaging with the language itself will be sufficient to give you fulfilment. This fulfilment will be necessary to spend the hundreds, eventually thousands, of hours you are going to spend with your chosen language.
Consider how important each of these categories of reasons are important to you and how the languages you are considering align which each of these. If you need to, write it down. By the end of this exercise, you should have a better idea which language you prefer.
One of the biggest factors that determine your success is if you stick with the language you choose. You are unlikely to learn a language you have no motivation to continue with. Consider if your reasons for learning are enough to keep you motivated. If not, you need to find a reason that will.
If there is some factor on your list that is important but you don’t feel it will motivate you to sit down and study, you need to consider why you feel that it’s important to consider in the first place. It may not be truly important to you.
Key tip: Spend some time trying them out
If you’re still undecided, I advise you spend a bit of time listening to and potentially even studying each. You could even spend some time learning about the languages as they relate to your priorities, such as culture, its usefulness in work, or what the difficult aspects might be. Any amount of time learning a language is useful for future languages because it helps you understand how language can work, and you will start to see similarities that make learning new concepts much easier.