(SOLVED) Translation doesn't work

I’m trying to extend the events plugin (https://github.com/kalebheitzman/grav-plugin-events ).

There are the following code snippets in calendar.html.twig:
{{ 'MONTHS_OF_THE_YEAR'|ta(calendar.date|date('n') - 1) ~ calendar.date|date(' Y') }}
and
{{ 'DAYS_OF_THE_WEEK'|ta(6) }}
which should translate the current month and the current weekday.

But on the resulting page I can only see
MONTHS_OF_THE_YEAR[8] 2019
and
DAYS_OF_THE_WEEK[6]

What's wrong with the Twig code? Regarding the manual this should work as far as I understand it.

Looks good 😎