There was a good improvement moving the spellchecker dictionaries to the default ones used by Chrome.
However, these dictionaries are not customizable and it is not possible to add languages not buit-in.
Then, a plugin that let users add custom dictionaries would help people who use uncommon languages (such as Latin), or if they already have their custom dictionaries with additional languages for their specific use cases in those languages existing languages (for example, by people who work in the medical field or others technical areas with their own lingo).