Time zones: the localized GMT formats

[Jon Zeppieri]

It's not clear to me how to determine the concrete format to use for
the localized GMT time zone formats. The description of these formats
begins:
===
Localized GMT format: A constant, specific offset from GMT (or UTC),
which may be in a translated form. There are two styles for this. The
first is used when there is an explicit non-zero offset from GMT; this
style is specified by the <gmtFormat> element and <hourFormat>
element. The long format always uses 2-digit hours field and minutes
field, with optional 2-digit seconds field. The short format is
intended for the shortest representation and uses hour fields without
leading zero, with optional 2-digit minutes and seconds fields. The
digits used for hours, minutes and seconds fields in this format are
the locale's default decimal digits:
===

While this is suggestive, it leaves a lot unsaid. The <hourFormat> for
"en" is "+HH:mm;-HH:mm". This looks appropriate to use for the long
localized format -- except that it's not obvious what to do with the
"optional 2-digit seconds field" mentioned above.

More troubling, however, is that in the general case, I don't know how
to generate the short form using this data. While it's easy to strip
out the "mm" portion of this when it's not needed, I don't know, in
general, how to deal with separators or possibly literal portions of
this pattern that should be removed along with the "mm." For example,
in this particular case, I know that I'd have to remove the colon
before the minute pattern, but I imagine that a locale could use
something like: "HH 'hours', mm 'minutes'," and I would not know to
remove the entirety of ", mm 'minutes'" without treating it as a
special case.

Is there a way to generate the short format for an arbitrary locale?
Also, how are optional seconds supposed to be handled?

- Jon