@straver, it's because of the change: "...Linux? Chrome will automatically display in the default system language for your computer." - source link.
As Opera is based on Chromium/Chrome code, it's the same for Opera for now.
You can try to run the Opera with the --lang switch as shown below:
.../opera --lang=nl
.../opera --lang=nl-NL
.../opera --lang=nl-NL,nl
(depending on which of the switch values works for you)

Small notice: the change that you made is as it says for the "websites" and not for the user interface.