what languages should i learn?

One language that I am planning to learn is russian. When it comes to a language my priority is: speaking/listening, reading, writing. Like i said, i'm more focused when it comes to communicating and talking to people so writing the alphabet isn't a first concern of mine.
Here are the list of languages I had in mind and I was wondering which i should learn or sounds the most useful. Any advice, experiences or resources will help.

mandarin - Its the fastest growing language and very popular so it may help me in the future when it comes to career or travelling?

French - I did it at GCSE which means I have some resources and can be easily familiarized with alot of content. Although i didn't like the lessons, I think it may be because of the school system and not the language itself so im willing to try again.
Spanish - Again quite popular, i like the sound of it too. I didn't do it for GCSE so thats a disadvantage.

Korean - This one is at the bottom of my list but i only put it down bc i swear korean entertainment like kpop is becoming more popular so it may be more easy to access resources and learn some words through more interesting ways? Thats a stupid reason but compared to other languages, it was a better excuse.

I am also a language enthusiast. With some of the languages that ou have listed being some of those that I too want to learn, I can say with reasonable confidence: you're brooding for a punishment.

If your mother tongue is English, it's said that some languages would be more difficult to learn than others.

Level of difficulty
The Foreign Service Institute in the US have ranked languages based on their difficulty to learn in relation to how close they are to English. See the following for example:
https://effectivelanguagelearning.com/language-guide/language-difficulty/
https://blog.rosettastone.com/the-complete-list-of-language-difficulty-rankings/
https://www.fsi-language-courses.org/blog/fsi-language-difficulty/

French and Spanish would be the easiest to learn. Russian is classed as Category 4, which isn't easy. Mandarin and Korean are in Category 5, the most difficult and the most dissimilar to English.

The thing with Korean and Mandarin is that they are tonal languages, so pronouncing a word in a different tone can mean something completely different (think Chris Tucker in Rush Hour 2's kareoke scene).

Your motivation
Then you should consider the reason for learning the languages. If you want to learn the language in order to work in the country that speaks the language, then it would make more sense to find out the national languages in the countries that you want to go to. If you want to learn the language because you want to speak to specific people or immerse in their particular culture, then the language itself is priority.

Widespread use of language
Mandarin is becoming popular because the ethnic Chinese population is growing. You don't get that many people outside of the culture that learn the language in my experience. See the following for example:
https://en.wikipedia.org/wiki/List_of_countries_and_territories_where_Chinese_is_an_official_language

Korean has more or less being relatively steady in my experience, but has been increasingly popular due to the recent rise in exposure to their media. See the following for its distribution as an official language: https://en.wikipedia.org/wiki/Korean_language

French is the second most widely spoken language in the world as a natural language. Unless you are interested in travelling to Canada (Quebec specifically), various areas of Africa, and a few countries in the Carribbean, then French is not that widely spoken in other countries.
Spanish is useful should you wish to travel round Latin America or go to Spain.
See the following maps:
https://en.wikipedia.org/wiki/Geographical_distribution_of_French_speakers
https://en.wikipedia.org/wiki/List_of_countries_and_territories_where_Spanish_is_an_official_language

Russian is only really an official language in former USSR countries, and I don't think they're ideal places to visit at the moment considering recent events: https://en.wikipedia.org/wiki/File:Countries_Where_Russian_is_Official_or_Widely_Spoken.png

Learning the language
Then there's the matter of which one you would enjoy learning the most and use the most. Learning a language is like a muscle; the more you use it the strong it gets, the less you use it the weaker it becomes. If you spend all of your spare time watching Korean drama, it makes sense to learn Korean first since you would get the opportunity to use it more than the others. If you speak with French people a lot more often, then I would prioritise learning French. If you're dating someone who is a Spanish speaker, I would prioritise learning that language before the others, for various reasons.

When comparing fluency of languages, I refer to the CEFR scale just to monitor my progress:
https://en.wikipedia.org/wiki/Common_European_Framework_of_Reference_for_Languages#Language-specific_scales
Should you ever want qualifications in the specific language, I use the above to see whether I am getting a good deal for what I am paying for. For example, I don't need a degree in the above languages in order to become fluent in them, and instead go straight for the official language certificates after completing the equivalent of A Level.

Should you wish to learn Spanish and French and get qualifications in them, I recommend doing GCSE and A Level courses in the languages as opposed to taking up any random course or program. This way, you would typically get more value for your money. Should you wish to take the exams and get the official qualifications in them, I think they are only available as International GCSE and International A Levels through EdExcel outside of sixth form; you can find online courses for these, but the exams you would need to book separately by yourself.
Should you wish to pick up the official A Level textbooks for these and study by yourself, you can do so usually with extreme difficulty.

Personally, my go to for simple stuff would be DuoLingo (and yes they have their own language certificates, but I personally don't take them seriously). The more difficult stuff that you can pick up would be the series by Michel Thomas, but he only covers the west European languages.
Then there are Languages for All courses at your local universities. These are courses that are commonly available to the public (for a fee) and you get a certificate of completion afterwards, should you pass. They tend to follow the same structure as those in the CEFR framework and they are the same courses that students at the universities take for their language degrees. They are unfortunately more expensive than GCSE and A Level courses, but you would get a classroom environment, so it's a bit of give and take.

With Korean, Russian, and Mandarin, the resources are more scarce. Whilst there are A Levels for Russian and Chinese, there isn't one for Korean.

You can also sometimes find Chinese societies in your local town/city (or within 30 miles of where you live) since they are pretty commonplace. From these societies, you can sometimes pay for lessons in the language as well as learn more about the culture.

In terms of learning a language, I have found the following video that would be of interest:
https://www.youtube.com/watch?v=illApgaLgGA
The method described above unfortunately requires you to find said elementary books and a native speaker to bug day in day out for about a month without them getting annoyed.

As one French speaker told me, the most effective way to ever learn a language is to immerse yourself in it by visiting the country where the language is spoken for 3-6 months. It's difficult to not acquire the language if you use it daily (having said that, I have met some people who have spent decades in a country and learned nothing about the local language because they have been so secluded or decided not to actively take lessons in it). It's definitely the more expensive option, but if you intend to said travel then it's something worth thinking about. (I don't recommend doing this for Russian at the moment though, and I wouldn't go to anywhere but South Korea for Korean.)
Do also learn about the local laws and customs before you go should you do this; you would never know whether you would be the next person who would be serving a 20 year prison sentence just because you were ignorant of a local law.
If you have difficulty keeping yourself financially afloat, consider teaching English as a foreign language as a means of funding your travels. The problem is that you should really know the local language to a fluent level before doing this, so it's a bit of a Catch-22. However, should you do I recommend getting either CELTA (https://www.cambridgeenglish.org/teaching-english/teaching-qualifications/celta/) or CertTESOL (https://www.trinitycollege.com/qualifications/teaching-english/certtesol) as your main teaching qualification since they are the most internationally recognised for British English (as well as being relatively cheap).
There are other ways of funding your travels e.g. online business, investments, freelancing, etc., but teaching English is the most common for travelers from what I hear.

For me, it makes more sense to hire a tutor after you have learned the basics as opposed to getting a tutor to learn everything (significantly more cost effective). Having said that, a tutor can 10x your progress since they tailor their teaching to your specific needs instead to a group of people.

Not sure if the above helps. If you have specific questions, let me know.

I think you need to ask yourself why a particular language appeals to you.

For me, I've chosen to learn French and Spanish for now, because:-
I would personally say Spanish is somewhat easier than French, as there are less pre-fixes and there no where near the same number of specific rules / protocols. IMHO, you'd find it a lot easier picking up Spanish with a basic French knowledge than if it were the other way round; despite the fact they're structurally similar. Also, French words need to be pronounced in a very specific way, whereas you can get away with more slip-ups in Spanish.

A word of advice though... the masculine and feminine nouns are not consistent across the two languages (it seems pretty random). For example, off the top of my head, the words car, bike, computer, bank and dress are the opposite genders in French and Spanish.

When I was doing my PhD, I was the only non-Asian in an office of 15 people, who all spoke Mandarin, so I picked up bits and pieces, I wish I took that opportunity to learn more when I had that chance. For the record, there are more celebrates who can speak (e.g. Mark Zuckerberg)... and looking at how big Chinese cars have become in the past few years, Mandarin is bound to become an important language in the business world, moving forward.

Anyway, check this kid out for inspiration... I swear to God she's gonna rule the world one day 😎

https://www.youtube.com/watch?v=KXGmt0dusdo