Check out this link as far as the Chinese languages. Mandarin is just part of the Chinese language
What do you think the most useful language is to learn? And which out of French, Spanish, Mandarin, and German would you take?
Deutsch, naturlich! Da ich schon Deutsch sprechen und Deutsch verwendet die zweite Sprache der Wissenschaft zu sein, sollten alle Menschen Deutsch sprechen.
Mandarin ... I'm so very impressed you didn't say Chinese.
In the US, the most useful second language to learn is Spanish. That way, you would understand the instructions in both languages.
After the international language which is English, Spanish is one of the most used language in the world, we have so many Spanish regions and they are so in use.
In order Spanish, French, German, before all, i'll attend for Italian! Loves it!
I offer a different take:
The language of mathematics is universal and probably the Most Useful of all. It is certainly the most pervasive.
Any of the languages you list are great choices to learn. Che mondo meraviglioso sarebbe se più persone parlano più lingue.
The choice depends on where you live and whether you plan to travel. Spanish has a lot of appeal but I'd never get to use it. In my neck of the woods I'd probably opt for German.
I recently downloaded an app to learn how to read and write Cyrillic and, inevitably, I'm picking up a few Russian words with it. Not the same as actually learning the language, but fun to do.
I'd go with French! I speak Spanish pretty well and know a little German, and though I understand French fairly well, I can't speak it as fluently as I'd like to. I've wanted to really learn Mandarin (I want to go to China!), but I gave up at some point.
Latin, and here's why:-
From the choice on offer I'd probably choose French and Spanish as I'm already familiar with those.
Based on Science Fiction shows about the future ... Or possible futures ... English and Mandarin.