Hello Coronado+38,
Thank you for using Apple Support Communities! I understand that the system language on your Mac is set to Dutch, however the Safari menu bar is shown in English. You've come to the right place for a resolution.
Here's a resource that could help:
macOS Sierra: Change the language your Mac uses
Let's start by making sure that your preferred language is set to Nederlands - Dutch as the primary language. Then go to System Preferences > Keyboard > Text. Ensure that Spelling is set to either Deutsch or Automatic by Language as well. Keep us posted on the results.
Cheers!