I want to keep only one English keyboard layout (US keyboard) but need spell checkers for English (United States) and English (Australia), probably English (United Kingdom) as well. When I added all flavors I end up with multiple entries for US keyboard layouts (this is what I use for all English types) in my list for input keyboards that mess up this list. I switch between other non-English languages and don't need this list to be any bigger than number of languages I use by merely increasing occurrences of same keyboard layout (or unneeded layouts).
In previous versions of Windows(95/98) long time ago I used a global spell checker called Tequita or something like that. That stop being compatible with newer Windows. They were not that efficient.
Later the spell checkers started going by application like MS Office, Browsers and more. Being add-ons, plug-ins or settings.
In the case of IE, I am aware of Speckie(free for personal use) being an add-on for that browser(read the compatibility info. on their download page): https://www.speckie.com/dload
In the case of other browsers like Chrome is a setting in this page: chrome://settings/?search=lang
In the Case of Firefox is an ad-on or extension depending on the version. https://support.mozilla.org/en-US/kb/how-do-i-use-firefox-spell-checker
In the case of MS Office, in prior versions was included in the installers. More on the newer versions below: https://support.office.com/en-us/article/add-or-edit-words-in-a-spell-check-dictionary-56e5c373-29f8-4d11-baf6-87151725c0dc
On Windows 10, whenever you're typing on the web using Microsoft Edge, or an app like Mail or OneNote (mobile), they use the Windows Language. You can add or modify to the Default Diccionary as per this link below: https://www.windowscentral.com/how-edit-custom-spell-check-dictionary-windows-10
As per prior Superuser link here: How to change spell check language in Microsoft Edge
Microsoft Edge does not support multiple spell-check dictionaries that are switchable on-the-fly. You need to change the keyboard language for that and also restart the browser, which is sort of useless as most people only have one connected keyboard.
This is a common complaint which Microsoft simply fails to understand, not understanding at all the needs of users dealing with multiple languages in their daily life.
Until Microsoft does something about this lack, the only solution is to use another browser.
As stated, goes by application. Some of us that use different Languages and/or Dictionaries want this to be simple, but as you can see is not.
I hope this somehow helps,
Please see section D in this Microsoft page: https://answers.microsoft.com/en-us/ie/forum/ie11-iewindows_10/spelling-correction-and-checking-in-internet/a1321c59-3623-49a2-8fce-388ecc2cb057
In Win 10 the spelling is built-in to Windows 10 rather than just the browsers.