Mandarin and French are likely to be the most useful. French because of how much territory they had influenced and Mandarin for business.

Python would be a great choice too 😉

Depends on why you want to learn the language. I'd say mandarin could be very useful to know for future employment prospects.

The language of wherever you want to spend substantial time. Probably not Russian I would think, but you do you

Back in reddit in r/languagelearning whenever someone asked this question without giving any context about who they are or what they want out of language learning there was a long running joke to reply with "Uzbek"

You should learn Uzbek.

当然是 汉语/普通话 啦

这建议不是因为我祖先是华裔，没偏见。😁

Of course its Chinese Language - Mandarin

My advice is in no way biased because of my Chinese Ancestry, totally no bias at all! 😉

(You shall suffer the same fate as me being tortured by the very difficult writing system)

But serious tho, if China internationalizes more, democratizes more. I can see a Democratic China starting to become equals with the, now-in-decline- USA in a fascist-ish downward spiral

Or, you know, you wanna become an astronaut then have to perform an emergency landing due to a space-disaster by commandeering a Chinese landing-capsule and doesn't understand the controls because they are all in Chinese, then its gonna be a lifesaver.

P.S. I haven't written in Chinese (besides typing pinyin) for over 10 years, so maybe you'll beat me, a "native" speaker, in terms of skill? 👀 (Too difficult, I gave up. Not in the country anymore, so its not too useful to me)

It it was my choice I would try to learn the language of the culture I'm most interested in

I'm fluent in Spanish and English, currently learning French. I like to describe it as Spanish with all the parts of English that make no sense. But knowing Spanish does make it a lot easier.

French will help you in Europe and Canada

Depends on your goals. I'd say learn the one you have the most use for. If you don't have a use and are just learning for fun, then pick the most interesting one. If you're learning intending to acquire the language as quickly as possible, then either Italian or French would be good choices.

I am learning Chinese (mandarin, aka 汉语, aka 普通话) and I personally find it very logical, interesting, and fun as a native english speaker who used to be conversationally fluent in Spanish. There are definite difficulties with the language (soooooo many homonyms, characters (汉子) take some getting used to, tones, etc) but if you learn it, there is a lot of reading material and media that will become accessible to you. Additionally some things about it are easier than other languages - like the grammar is very simple, you don't have to worry about conjugations and tenses as much. Also, I think that it provides more cognitive benefits because of how different it is to romance languages.

汉语是很酷。我推荐你！

Would choose Toki Pona, later I would like to learn Swahili too

Why these languages specifically? If it's for different reasons you should pick what's most important to you. Also, with Spanish and Portuguese, Italian and French should be much easier than the others but don't you already understand a good part of them?

Esperanto is Chad language

English, spanish, Portuguese... they all shear a lot of common words. With no context and thinking of "which language would be more fun to learn", I would choose between ukrainian, arabic and mandarin.

French and Italian will come relatively easy for you, the others are quite different. Personally I'd go with Arabic, but that's just because there are many Arabic speaking people at my job.

French is probably the most wide spread. Mandarin has most people, but only in one country.

I've been learning Russian and it's so much fun. Very challenging, but not as much as I expected. Of course, the case system is quite different from English, so you would have an easier time with French or Italian.

I've also heard Arabic is very cool to learn. It sounds quite nice too.