Fillin Forge hands every translatable string to a filter and asks which language is active, so a translation plugin such as Lang Forge does the translating. The hooks are listed in Translation String Hooks.
formforge_translate_string(filter): receives$string,$key,$field_idand$form_id, and returns the string to show. Without a listener the original string is shown.formforge_current_language(filter): receives an empty string and$form_id, and returns the language code of the page.formforge/strings_registered(action): fires when a form is created or updated, with the form id and the list of its strings.