You'll find all the various i18n.properties files in the source code, in the source/sbs_employee folder. They contain the global list of strings defined for each language that SBS supports (jive_i18n_en.properties for English, jive_i18n_de.properties for German, etc)
You can override these strings by putting them in a plugin, in a file called plugin_i18n.properties inside your plugin.
You can find more information about plugins in the developers community, and we also cover this in our developer training course:
