Good evening Daniel and welcome back!
Thank you for reaching out to us.
I would love to help you with this topic.
What is going on is that WordPress plugins only include po/mo files for “other” version of English (non US – en_CA, en_AU, en_GB).
In other words, the strings contained in the plugin itself are instead of a en_US.po and en_US.mo.
With that in mind, if you do create a the-events-calendar-en_US.po (and its attached .mo), that will work AND have precedence over the plugin’s strings. An easy way to do this is to rename the-events-calendar-en_GB.po for example).
I highly recommend reading the 3 following articles on translation:
Let me know how that goes.
Best regards,
Geoff B.