Thank You very much for your Calendar! I just installed last version with phpBB 3.2.5 and liked it very much!
It looks like You made a great progress during last year!
I've caught several bugs, i'd like to report.
1. Minor: There is one not localized word 'entfernen' when you select type of an appointment. See main_listener.php line 1255
2. Micro: as for me "before Easter" sounds a bit better, than "until Easter". Google agrees 1.3B vs. 300M
3. Serious: I tried to say (don't ask why!) it would be -5 (minus 5) of us, attending event, and got
Code: Alles auswählen
SQL ERROR [ mysqli ]
Out of range value for column 'number' at row 1 
INSERT INTO phpbb32_calendar_participants (POST_ID, EVENT_DATE, USER_ID, NUMBER, PARTICIPANTS, COMMENTS, DATE) VALUES ('11', '', '2', -5, 'no', '', '2019-1-16-00-42')
4. Wow! told i, modified html attribute via browser inspector and took 9999999999 (almost 10B) friends with me, and got very similar error
I suppose, min(100, max(0, $num_friends)) will fix both issues.
But i still like your Mod very much!
I think, i'll make russian localization for it.
If you have github or similar repo, i'd like to make a pull request to it.
Or i can send you localization, if You'd like.