The function sets "the current language" to name.
The current language value affects results of "language-local" and
"lowercase" mapping procedures in encoding and decoding conversions.
If name argument is NULL, the current language is determined by
For example, the locale category LC_CTYPE has been set to "ja_JP.UTF-8",
the idnkit library supposes the current language is "ja" (Japanese).
If an application changes its locale setting, also the current language
value is changed automatically.
However, once a non-NULL language name has been set by
idn_setlanguage() or set by "language" entry in a configuration
file, the current language will not be changed by locale information
Upon success, idn_setlanguage() returns idn_success.
Otherwise, it returns an error code.
See idn_result_tostring(3) for the complete list of error codes.