Function strftime() use the locales installed in your system (linux).
If you are like me and only leave in the system the locales you use normally (en_US and your own language locale, like es_ES), you'll only be able to use the locales installed. If your application is translated to other languages, you need these locales too.
The name of the locale in your system is important too. This can be a problem when you want to distribute the app.
If you have this locales in your system:
and use setlocale('es_ES'), the result will use the iso-8859-1 charset even if you have all your system, files and configuration options in UTF-8. To receive content in UTF-8, in this example, you need to use setlocale('es_ES.UTF-8') or setlocale('es_ES.UTF-8@UTF-8').