Localize long number strings

Summary:
Localize long number strings so that they get the proper locale-specific separator.

This is based on @broulik's old patch from https://git.reviewboard.kde.org/r/127800

BUG: 409077
FIXED-IN: 5.62

Test Plan:
Unit tests pass.

With US English locale, before:

With US English locale, after:

Reviewers: Localization, Frameworks, broulik, aacid

Reviewed By: Localization, aacid

Subscribers: aacid, huftis, safaalfulaij, mikeroyal, aspotashev, ilic, kde-frameworks-devel, broulik

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D22069