HackMD spell check language inconsistent with browser settings

### Is this a hackmd.io issue?

- [X] Yes, the bug happens on https://hackmd.io

### What's the problem

Incorrect spell check language / no option to set spell check language for document

#### Current behaviour

My spell check language is defaulting to English (US), and there doesn't seem to be a way to change this (my browser is sending the `Accept-Language: en-GB,en;q=0.5` in request headers).

Provide steps to reproduce:

1. Set browser language preference to prefer a different language
1. Type a string with a non en-US spelling e.g. `color colour`

#### Expected behaviour

Select a spell check dictionary based on users' browser preferences. Optionally allow the owner to set a document-wide preference.

#### Environment

I use HackMD on:

**Desktop**
- OS: Linux
- Browser: Firefox
- Browser Version: 75.0

#### Additional context

Add any other context about the problem here.