I had a choice to study a foreign language, I took it and never
regretted it. I found language classes a lot more interesting than
my core modules in school. 
Passion is of the utmost importance I feel. I thoroughly enjoyed
the learning experience. Besides, it's not easy picking up a whole
new language, so interests again is very important!
Japanese is probably one of the more popular foreign languages here
in Singapore. So generally speaking, it's won't be too difficult
finding resources.
Whether or not it gives you an edge in finding a job, logically
speaking it depends on the type of job. The biggest misconcept is
that Japanese companies based in Singapore will give the edge to
candidates who speak their language. That's not true. Cos simply,
you don't need to know japanese to work in Singapore! Unless you're
talking about a job based abroad, knowing the foreign language will
certainly give you the edge over others who don't!
As for the best choice of language, I guess it depends on where
your interests lie and if you really want to make use of the added
knowledge in your career path. I opted for Spanish. It was purely
for interests. I wasn't going to use it to apply for a company
based in South America.
Lastly, I feel that in the long run, it could give you more career
alternatives abroad, in your case Japan. But again, it only gives
you the extra edge only if you have the basic qualifications and
experience. From what I know, very few companies employ local fresh
graduates or newbies for overseas assignments straight up.
All in my humble opinion. Hope it helps! 
