[SystemInfo] fix the language and country syntax

BUG=XWALK-2361
Function name too common to understand and I'd prefer to make this function as inline. Suggest as
```
std::string GetFirstSubstringByDot(const std::string& str) {
  return str.substr(0, str.find_first_of("."));
}
```

For the places where to call, use 
```
language_ = GetFirstSubstringByDot(std::string(language_info));
```