A new language will probably not help you to find a job so I would not do it for that purpose. There are only few jobs where speaking an additional language is more than nice to have. They hire you for a specific function in a company and unless this function is translation, it is going to be not that beneficial. Not mocking this at all, it is great to speak different languages, I am actually thinking about learning another one myself, but in terms of jobs languages are vastly overrated. Most employees that deal with international customers have to speak English.
In terms of learning it fast, I would do language courses until you have a basic level and then continue with these "tandem-things" (maybe they are called different in the UK), where you meet with a Chinese that wants to improve his or her English and you speak with each other. One Chinese PhD student in my lab is doing that to learn German. Doesn't cost anything and is really efficient. I think mandarin is too complex to learn it on an online platform. Good luck.