Definitely Spanish, it sounds so poetic if that makes sense lol and I think it would be really useful considering how it is spoken in half of Central and South America and also in Spain ofc (which is a beautiful country, I actually visited Barcelona and it is number one on my list of European cities I would like to live in)
Next is Arabic, I actually think it sounds beautiful and Arabic script literally looks like art. Also I like Arabic culture and I would like to visit more Arabic countries.
I would also like to speak Russian, it sounds so bold and powerful.
And the last language on my list would be Japanese, it is so cool and not a lot of people outside of Japan know how to speak it. Also I like to watch anime. 