Moreover, since the unsupported locale is stored in localStorage, GWD won't start even after you change the locale setting; you also have to clear GWD data as well.
GWD should fallback to the next preferred locale in the list or, if there aren't, to English.
To fix, open terminal and type exactly:
rm -r .local/share/google-web-designer
Then, to run GWD:
It will be started in english, you can now start it as normal.
|fil-PH, tl, tl-PH||Filipino|
|he*, he-IL, iw-IL||Hebrew|
|id-ID, in, in-ID||Indonesian|
|no-NO, nb, nb-NO||Norwegian (Bokmål)|
|sr-RS, sr-Cyrl-RS||Serbian (Cyrillic)|