seems like an API & ABI break; maybe we can't implement this suggestion?

`g_autofree gchar *locale_noenc = NULL;`, otherwise we'll crash if `strv == NULL`.

Can you throw the whole code of this function behind a GLib version check macro, so we fall back to the GLib-internal version as soon as it is available with newer GLib? That also makes the code self-explanatory and is easy to grep for once we do raise the minimum GLib version. See `as_date_time_format_iso8601` for one such example: https://github.com/ximion/appstream/blob/master/src/as-utils.c#L1744-L1782 (there's quite a few more in AppStream's code)

Brace should be on the end of the previous line to match existing style.

`@search` > `@find`

Nit: added an extra newline here.