Put correct decimal place separators in exercises

Summary:
Use the correct localized decimal place separator in the basic
addition, subtraction, multiplication, and division exercises. Also, allow
changing the icu language at runtime using a lang=LANG parameter.
Fixes T1141

Test Plan:
Visit all the _decimals exercises, make sure they still work. Visit
the exercises with lang=fr, make sure that the decimals are separated using a
comma instead of a period.

Reviewers: eater, alpert

Reviewed By: alpert

CC: james, alpert

Maniphest Tasks: T1141

Differential Revision: http://phabricator.khanacademy.org/D3386