Like Anonymous Monk says, a failure in locale.t can most likely be ignored, as you shouldn't be using locales anyway and the locale support across many OSes is inconsistent and/or buggy.

May I ask why you are compiling 5.12 specifically? The most recent release is 5.24, which is officially supported. Unless you are doing this to match an existing installation, I would go for a more recent version. If you are doing this to match an existing installation, you are taking the correct approach.

If the locale failure worries you, consider compiling a more recent version just to see if the error goes away there. If you are running a less common OS and want to improve Perl support for it, and the error still happens with the most recent releases of Perl, you can send the relevant information using the perlbug utility from your compiled-but-not-installed version of Perl to send all relevant information to the Perl 5 Porters.