Hello again Karoliina,
Those “built in” stings you should be able to translate with the help of the .po translation files. You can find them for example in wp-content/plugins/the-events-calendar/lang folder.
You can edit them with a software like PoEdit or a plugin like Loco Translate.
Note, the best place for the .po / .mo files is the wp-content/languages/plugins folder, because there they will not be overwritten with a plugin update.
I hope this helps you solve this. Let me know if you have any more questions on this topic.
Cheers,
Andras