In the C locale, GNU programs should stick to plain ASCII for quotation characters in messages to users: preferably 0x60 (‘`’) for left quotes and 0x27 (‘'’) for right quotes. It is ok, but not required, to use locale-specific quotes in other locales.
The Gnulib quote and
quotearg modules provide a reasonably straightforward way to
support locale-specific quote characters, as well as taking care of
other issues, such as quoting a filename that itself contains a quote
character. See the Gnulib documentation for usage details.
In any case, the documentation for your program should clearly specify how it does quoting, if different than the preferred method of ‘`’ and ‘'’. This is especially important if the output of your program is ever likely to be parsed by another program.
Quotation characters are a difficult area in the computing world at this time: there are no true left or right quote characters in ASCII, or even Latin1; the ‘`’ character we use was standardized as a grave accent. Moreover, Latin1 is still not universally usable.
Unicode contains the unambiguous quote characters required, and its common encoding UTF-8 is upward compatible with ASCII. However, Unicode and UTF-8 are not universally well-supported, either.
This may change over the next few years, and then we will revisit this.