Many years ago, I had a simple grep-based script that was used for cleaning up strings for Moodle 2.0. The relevant issue is MDL-16769. The script is available at https://github.com/moodlehq/moodle-local_amos/blob/master/bin/detect.sh and may be used as an inspiration for a custom solution. Please feel encouraged to share eventual improvements.
General developer forum
Unused language string detection
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