Use platform.language more

[rzhao271]

Ref #169114

I added the platform.language variable in more parts of the code. That variable allows functions such as .toLocaleString() to use the application language instead of the system language. The PR also changes the terminal to use the platform.language variable, because in older versions, it was using the application language rather than the system language to create a valid locale to use.

Some examples of toLocaleString for reference:

> const n = 5;
'5'
> n.toLocaleString('en-US')
'5'
> n.toLocaleString('en-US.UTF8')
Uncaught RangeError: Incorrect locale information provided
    at Number.toLocaleString (<anonymous>)
> n.toLocaleString('en_US.UTF8')
Uncaught RangeError: Incorrect locale information provided
    at Number.toLocaleString (<anonymous>)
> n.toLocaleString('qps-ploc')
'5'
> n.toLocaleString('qps-ploc')
'5'
> n.toLocaleString('zh-hans')
'5'
> n.toLocaleString('zh-hans-cn')
'5'
> n.toLocaleString('zh-cn')
'5'

platform.language has the form zh-cn, qps-ploc, en, etc., so we should be good for the .toLocaleString() calls.