ResXManager supports spell checking by using the default .Net spell checker. In order to enable spell checking for any language, you must have the corresponding .Net language pack installed.
You can find the latest language packs here: https://support.microsoft.com/en-us/help/4054529/microsoft-net-framework-4-7-2-language-pack-offline-installer-for-wind
In Win10 you have to install the language packs in Settings/Region & language:
- It will only work if all options are installed for the language.
- You need at least Windows 10 Creators Update (version 1703), in earlier Win10 versions it does not work at all.