Refactor strftime() to use langinfo constants for locale-based formatting :

- replaced hardcoded day and month names with calls to nl_langinfo,
- integrated langinfo constants  DAY_1, ABDAY_1, MON_1, ABMON_1) for weekday and month names,
- used AM_STR and PM_STR for AM/PM strings in %p and %P specifiers,
- add comments.