From 3.3.0/4.3.0 version, the resource files per language are checked in this order:

• Portal specific adoption <filename>.<locale>.<portalid>.resx (e.g. user.ascx.de-DE.0.resx)
• Host specific adoption <filename>.<locale>.Host.resx (e.g. user.ascx.de-DE.Host.resx)
• Standard Language pack <filename>.<locale>.resx (e.g. user.ascx.de-DE.resx)

.<locale> is omitted for default language ("en-US")

Portal and Host specific adoptions only contain keys, that values differ from the content in the standard language pack file. AFAIK adoptions cannot contain keys, that are not listed in the System language pack.

Leupold said that the host and portal files can only override keys, not add keys wich means the key still has to be defined in the normal file

Wich will be overwritten again and again when upgrading, so the problem still exists

The only good solution this far:

• make your own profile editing module specially customized to your site's need's (not as hard as it sounds)
• Put this module on a hidden page
• change the *user page* setting, located under advanced site settings (replaces default profile page with your custom page)

Sindce this is a custom module, you efforts will not be overwriten when you upgrade the core

This is the way I am going with my projects