Depends on my reasons.
I'm fluent in four currently. Grew up bilingual & picked up the other two & a few others over the years.
If i'm trying to impress people & be a hipster smart ass I suppose i'd learn a smattering of many languages.
I' think it's better to be very good at one language & then use that language to give you the foundation to be fluent in another & so on.
My reasons for learning a language:
1) I can think in another culture's viewpoint. Understand why people of a certain culture express themselves a certain way.
2)It's just adding another personality.That's Jason Bourne level cool
3) Fun
4)Relationships (women, business etc)
If you're fluent in English & you want to leverage on that, why not start with Spanish?
1) entire South American continent speaks it
2) Useful if you ever decide to work with the United Nations, heh.
3) may make it easier if you ever want to use that to leverage learning other Latin/Romance languages like Italian, Portuguese or French.
4) access to potentially gorgeous women
Or, you could go with Mandarin, if you plan on heading East.
1) teaches you the concept of a tonal language
2) no pesky tenses or conjugation, yeah!!
3) seems difficult but there's a fairly logical structure to it. Once you grasp the pattern, in some ways it's easier than English.
4) Can be leveraged (not a lot) to learn Cantonese if you're spending significant time in Hong Kong or Macau or Malaysia.
With Arabic (Middle eastern or North African?), i'd learn it if i was spending significant time in those entire regions or I wanted to work in Western intelligence or the Oil & Gas industry.
Arabic & certainly Russian would be on my bucket list for languages.
To answer your question, i'd encourage being very fluent in one language first instead of diffusing your effort. To be truly fluent, you need to be able to think in that particular language. To express emotion, impulsively curse, etc in that language.
You need a strong reason to immerse yourself in a particular language. Strong as in you get bagged & dropped into Russia or Eastern Ukraine & you're forced to know Russian type strong.