And of course it crashed (same event I opened this thread after, adding a translation): fatal error involved appParameters.php and bootstrap.php
That's an interesting thing. I thought I'd check this /var/cache/dev/appParameters.php file so I tried to download it, but Filezilla could not start transfer. I checked permissions and all nine boxes were neither checked nor empty but filled, numeric value was xxx. I set it to 775 and site is running again.
Now every time I change the translation I have to do the above again and again.