<note>Note that the first time this function is called, it calls
"setlocale (LC_CTYPE, nullptr)" to fetch current locale. The underlying
setlocale function is, in many implementations, NOT threadsafe. To avoid
problems, call this function once before multiple threads can call it.
This function is only used from harfbuzz.global.bufferGuessSegmentProperties by
HarfBuzz itself.</note>
Fetch the default language from current locale.
<note>Note that the first time this function is called, it calls "setlocale (LC_CTYPE, nullptr)" to fetch current locale. The underlying setlocale function is, in many implementations, NOT threadsafe. To avoid problems, call this function once before multiple threads can call it. This function is only used from harfbuzz.global.bufferGuessSegmentProperties by HarfBuzz itself.</note>