If anyone stumbles upon this issue in the future: upgrading from Moodle 3.0.x to Moodle 3.1.x solved it. I don't know if something was messed up in my installation and a version bump fixed that or if some fix was introduced in Moodle 3.1, but now cron output if fully in English.
General developer forum
Cron output - changing the language from local to English
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