I tried a `MessageFormat("a_{0,plural,other{num=#'#'=#'#'={1,number,##}!}}_z")` with values [7|15,].
I expected "a_num=15#=15#=7!_z" but got "a_num=1515=1515=7!_z".
I verified that the `insertFormattedNumber()` function handles `{}` (hence the nested `{1,number,##}` works fine) but ignores apostrophes.
Part of ticket #8319 implementation.
See TestMessageFormat.testApostropheInPluralAndSelect() & TestMessageFormat::testApostropheInPluralAndSelect().