If you call this with argument "unused=true" it searches for the translation strings that exist in en_US but are never used.
IMPORTANT: that can take quite a lot of time (up to 10 minutes), you need to tune the max_execution_time on your php.ini accordingly.
Can't scan workdir = /home/citrus/team/dolibarr-4.0.1/dev/translation/../../htdocs/langs/en_US/