How to add a supported language to the CQ / WEM

1) You need to overlay/overwrite /libs/cq/security/widgets/source/widgets/security/Preferences.js to change the list of available languages (used in security admin)
and overlay /libs/cq/security/content/tools/userProperties (inside change language list under items/common/items/lang/options - similar dialog to Preferences.js) (used for user drop-down in the top right on admin consoles)

2) Then you need to add the new language somewhere in the repo, e.g. /apps/yourapp/i18n, following the layout as under /libs/wcm/core/i18n (see also the above section on the translator UI). Stuff under /libs must not be changed!

3) Here is a sample package:
cq-i18n-add-language-sample.zip

4) adds /apps/test/i18n with translation of "New..." in zh-cn ("simplified") and zh-tw ("traditional") languages
overlays /apps/cq/security/content/tools/userProperties and /apps/cq/security/widgets/source/widgets/security/Preferences.js to add the zh-tw language

5) to check, set the current user's language to "Traditional Chinese" and look in the siteadmin: the first button in the toolbar should now be "traditional" instead of "New..."

Thanks Alexander Klimetschek from Adobe for this information.