B.Tech Mechanics

Yes, of course you should learn Japnese, if possible to grow your career. Engineers from the city often have opportunities, make work trips to Japan and vice versa. Knowing the native language of a country is always an advantage, as it helps you connect well with these people. The company gives you an interpreter, but you can connect better with people, if you know the language. Besides, a lot of authorities from Japan come to visit their firms here, and if you know the language, you can interact properly with them too. While school and college students opt to learn French and German languages, Japanese is catching up steadily. It was observed that about 90% of the students who enrolled for the Japanese speaking course in the country, have an engineering background.

There are a lot of options for mechanical engineer graduates. I am writing this answer because I am also a mechanical engineer. Basically, after doing mechanical, you can apply in IIMs, IAS and all other civil services exam ,but if you are really interested in the mechanical engineering then i will appreciate you to choose mechanical engg.
Now, come on the core mechanical engineering career options:
Basically, you will learn three type of courses in B.Tech:
1) Manufacturing related (material science, manufacturing science, solid work, auto cad, etc.)
2) Thermodynamics related (heat and mas transfer, thermodynamics, power plant, refrigeration and air conditioning, fluid mechanics, etc.)
3) Structural related (mechanics of martial, fracture mechanics, finite element method, theory of machine, etc.)
So, you should decide first that in which field are you interested?
If you want to join the PSU (Public Sector Unit), then their are lot of vacancy come each year in PSU for mechanical engineering.