OS-wide spelling checkers
System wide spellers for Windows are available as part of the tools installed via the Divvun Installer.
Enabling: Go to Settings > Devices > Typing, and turn on Highlight misspelled words. Further details can be found e.g. here. Remember that you also need to enable a keyboard for the language you intend to type in.
Usage: Just switch to the keyboard/language you want to type in, and the speller will automatically be triggered.
System wide spellers for macOS are available as part of the tools installed via the Divvun Installer.
To verify that the speller is working, open System Preferences > Keyboard > Text, and verify that your language is listed in the menu Spelling with «(MacDivvun)» after the language name. If not, restart your computer.
Usage: In applications using the system spelling checker services (most apps), just open the spelling checker dialog (Edit > Spelling and Grammar > Show Spelling and Grammar), and select your language in the language menu. If you always want to spell check in this language, you can make it the default spell checking language in System Preferences.
OS-wide spellers for Linux have to be built from source, but most of the required packages are available as standard, or easily installable for the various distributions. Further instructions can be found here, together with links to source tarballs for the supported languages.