Why some pages are still in English
English is the original. We translate in waves. A Dutch or Spanish header with an English article usually means that article has not been translated yet, not that your setting is broken.
Help articles on this support site are written in English first and then translated. Most articles are available in many languages, but a new or recently updated article can stay in English for a while.
If a whole site chrome (buttons, menu) is in the wrong language, see I am seeing the wrong language. If one sentence is badly translated, see How to report a bad translation.
