the ability to do well in learning a language really depends on how passionate and devoted you are to learning it imo. So pick whatever language/culture you find most interesting and go from there. As for the business perspective, I'd say Chinese or Spanish are the go to languages if you want some sort of international relevance.
Law firms do look favourably upon candidates who speak more than one language but it depends on relevance to that company. Like if you are required to negotiate with a Korean law firm or clients, knowing fluent korean would be advantageous. Same with Chinese. Languages useful in contemporary times are most likely Mandarin, Malay, Japanese, Korean etc. especially for the Asia-Pacific era.
Languages like French, Spanish etc. aren't so much.
However, ultimately take a language that will interest you because there's no point studying Korean or Malay if you have no passion or drive for the language. Knowing a second language, however is a useful asset in any job.
If you were to be deployed by a law firm to Russia for example, Russian would be useful to learn.