Most practical ones are English and Putonghua. But I think you know them and so you don't need them.
You should try Japanese in my opinion as Japan is getting stronger and stronger. There will be more chances to be in contact with them in your work. It is more practical.
French, German and Spanish are commonly spoken in the Europe. Spanish is spoken in Latin America and South America while French is an official language of Canada, too. It all depends on your interest and your work. But I think learning European languages, apart from English, is for interest mostly if you work and live in HK. I think Spanish is more commonly spoken in the world.
If you are really interested in language, I will say you can learn French first, and then Latin, Greek and Hebrew.
Arabian is another good choice if you are interested in language. It is very different from what you have been in contact if you haven't learnt that before. Indian is too complex because there are too many dialects/variations just like Chinese dialects.