make System language option work on macOS (fix getOSXSystemLang)

[akirashirosawa]

Summary

SUMMARY: None

Purpose of change

System language option does not work on MacOS. CFStringGetCStringPtr always return NULL. In Apple docs, I found that:

The functions that end with “Ptr” either return the desired pointer quickly, in constant time, or they return NULL. If the latter is the case, you should use CFStringGetCString.

https://developer.apple.com/library/archive/documentation/CoreFoundation/Conceptual/CFStrings/Articles/AccessingContents.html

Describe the solution

Add CFStringGetCString if CFStringGetCStringPtr returns NULL.
Also, I replaced reinterpret_cast with static_cast. Don't really know why reinterpret_cast is need here, if someone knows - feel free to change.

Describe alternatives you've considered

… if the code in question is not that performance-sensitive, you could simply call the CFStringGetCString function without even attempting to get the pointer first. (c) Apple docs

Testing

System language works correctly for MacOS.

Additional context