In Windows 10, there's the option to turn off autocorrection of spelling errors in "Settings - Devices - Input". In Windows 10 Mail, you can also disable the highlighting and autocorrection of spelling errors when writing an e-mail in the options menu. Still, the autocorrection will be "correcting" errors on its own, which is even more annoying when you're writing in a language Win10 doesn't understand.
I presume that the spell check is somehow integrated into the keyboard language, but only active within some applications (maybe those that don't appropriately control it). I have several different languages installed, with German being the main one, but no English keyboard installed, as it is simply not neccessary when using the german one (no additional keys/letters in English, and I prefer the language list not to get too long). Now when writing a mail in English, it autocorrects everything german-style (in Mail). Within the keyboard settings (control panel - time, language and region - language - keyboard=>options) I have access to some details regarding the spell checking (differently for different keyboard languages), but no option whatsoever to just turn it off! Regardless of me using a german layout for english input, there should be an option for anyone not wanting a spell check to turn it off, shouldn't it?
Does anybody have a clue whether there is another hidden option for spelling checks which could really turn this thing off?