If you're using Chrome, you can enable spellchecking for multiple languages.

1. Go to chrome://flags/#enable-multilingual-spellchecker and enable it.
2. Go to chrome://settings/languages and add the languages you want spellcheck for, then click "enable spellcheck for this language."

I haven't tried it, but from what I understand, Chrome will try to auto-detect what language you're typing in and use the correct language dictionary for whatever language you're typing in.