if you edit resource files from admin > language, a unique resx file will be created for that portal, containing only your changes. These will never be overwritten

Also, if you edit through host > language, if you select "host" instead of "system" for mode, you will create host specific translations that will also never be overwritten during upgrades...

Declic,

The language editor has 3 modes: system, admin and host. Admin is available from admin menu, both others from host menu.

• System is for creating distributable language packs and modifies default resource files
• Host is for adopting language files for all portals of the installation
• Admin is for customizing language files for a single portal

Host and admin create additional resource files, that only contain modified entries and do not get overwritten during upgrades.

Declic,

• If you use the language editor in "host" or "admin" mode, you get customizations, that do not get overwritten on installations of the module or any language pack
• If you use the language editor in "system" mode, your modification of the "en-US" locale get overwritten, when an update of the module is installed. Your modifications of the "fr-FR" locale get overwritten, when a language pack is installed, that contains a translation of this module.

HTH