This feature is available starting from the Essential plan.
Sometimes translators prefer to translate offline using familiar CAT tools. Instead of granting translators with the download/upload rights, use Offline translation project setting (project settings > General > Offline translation).
Once enabled, any user will see the offline translation widget in the Editor while in bilingual editing mode.
Users are able to download XLIFF of current target language, translate it offline and upload the translated XLIFF file over.
Uploading the file will replace all existing translations with those in the file being uploaded.
Filename and platform attributes of keys will not be changed.