Actually Electron natively supports multi-language spell check. SpellChecker | Electron
Chrome, Edge and another browsers based on Electron support it. Also other note taking apps that based on Electron support it, like Obsidian or Standard Notes.
I found it weird that Joplin don't support it, since it's uses Electron native spell checker. There other thread in the forum about this Multi-language spell checker? and Per-note language tag for spell checking.