For languages other than English, adding this line to your options.txt file:
"log_untranslated" : "1"
gets you a file "untranslated_messages.xx.log", where "xx" is your language code. So "untranslated_messages.fr.log" for French, "untranslated_messages.de.log" for German.
The log file contains lines like these:
msgid "Savefile"
msgstr ""
msgid "Go\nBack"
msgstr ""
The log file contents can be used in two ways. You can add them to the translation file, for example "foldit.fr.po", and place the new translations in the msgstr values. You can also add them to the translation template file, "foldit.pot", and use them in a new or updated translation.
Adding to "foldit.pot" can be helpful if you're using translation software like Poedit, which can match a template file to an existing translation.
Both the translation files and the outdated translation template file can be found under the most current resource/locale directory, for example:
Foldit/cmp-resources-0f03be3a8113157798a6335f2a45a51c/resources/locale
The translation file is read in when Foldit starts, so you must restart to see new translations.
If you update a translation file, be sure to save a copy outside of Foldit. The resources directory may be replaced when there's a new release, in which case your changes would be lost.
Right now the system for sharing translations on the Foldit website is broken. You can share translations as replies to a feedback, however. To do this rename (for example) "foldit.fr.po" to "foldit.fr.po.txt". Others can then download the attachment, remove the ".txt", and install it under resources/locale.
Of course, you can also share your translation outside of Foldit. The wiki doesn't like ".txt" files, but you can share on Discord quite easily.
The system for updating the translation template file, foldit.pot, also seems broken. The one in resources/locale hasn't been updated in several years. The untranslated messages log is an easy way for translators to find new messages to translate.