Thanks Tor-Bjorn, you are a hero! ??
Now it seems like all the errors are gone, except two:
– The “Details” button in the portfolio section isn’t translated.
– The “Related Projects” headline on the porfolio pages isn’t translated.
My guess is that this problem derives from the Filterable Portfolio plugin, rather than from the theme. There are fields in the portfolio settings for changing/translating these phrases, but they don’t seem to work.