Dear all,

There are a few strings in the `argparse` module which are not translatable through the `gettext` API.

Some have already been reported:
- the "--version" help text at Lib/argparse.py:1105 (reported in issue 16786, fixed by PR 12711);
- the "default" help text at Lib/argparse.py:697 (reported in 33775, fixed by PR 12711).

However, some others remain:
- the "default" help text for `BooleanOptionalAction` at Lib/argparse.py:878 (which, incidentally, will be duplicated when used with `ArgumentDefaultsHelpFormatter`);
- the "argument %(argument_name)s: %(message)s" error message at Lib/argparse.py:751;
- the formatted section heading at Lib/argparse.py:225: if the heading itself is translatable, the string "%(heading)s:" is not. More precisely, the colon right after the heading might also require localization, as some languages (e.g., French) typeset colons with a preceding non-breaking space (i.e., "%(heading)s :"). (Okay, I'll admit that this is nitpicking!)

I'll submit a pull request with proposed fixes for these strings.

Kind regards,
Santhosh