This does not appear to be easily possible.
Concrete 5.7 uses Zend Translate, and others on S.O. have asked about creating overrides for this (see override gettext .mo files).
If you only need to override a few words from the default US language, you can:
- Create a custom MO file using a tool such as POEditor.com, including just the words/terms you want to override
- Save the
.mo
file to /application/languages/[LANGUAGE-CODE]/LC_MESSAGES/
- Enable your new language file via C5 Dashboard > Settings > Basics > Languages
However if you need to override words from another language, it looks like you might have to create your own custom PO/MO files that contains the entire dictionary. You can probably import an existing one from the official c5 translations repository on Github.
This appears to be a major limitation of the Zend Translate library.