Can I earn a higher salary if I speak more than one language?

As bilingual and multilingual employees are higher in demand than those who can just speak one language, they can earn up to 10 to 15% earn more money on average than monolingual speakers.

Will my memory improve if I learn another language?

Yes. When switching between languages you are constantly digging into the memory bank which you have wisely expanded.

What are the most valuable languages to learn as an English speaker?

“Japanese and the Chinese languages were named the most lucrative non-English tongues for UK job seekers” according to this article - followed by French, German and Arabic.