Top 3 foreign languages that can greatly enhance your earning potential are:
Chinese (Mandarin)
There is much scope for employment in Chinese companies, both within India as well as overseas. As per the Confederation of Indian Industry (CII), in the past 15 years, 150+ Chinese companies have invested over $30 billion in India in different sectors such as consumer electronics and automotive industry.
French
Learning French can open up many opportunities for you. French is widely used in theatre, fashion industry, visual arts, as well as the hotel industry. Moreover, a good knowledge of French is useful for getting grants for enrolment in various postgraduate courses.
German
Learning German can get you great job offers. German companies employ many worldwide. German companies alone account for around 700,000 jobs in the U.S.