OS X
There’s no extra setup on macOS, you could enable/disable spellcheck from Edit → Spelling and Grammar on menu bar (like other apps).
Windows/Linux
You could open spellcheck panel by click Edit → Spell Check… from menu, or just hover on status bar and open the “spell check“ icon.
You could change the default spell check language or disable spell check by clicking “Global Setting…“ from the spell check panel or find it directly in preferences panel.
Windows 10
On Windows 10, Typora may use the system built-in spellchecker, you could enable “Highlight misspelled words” in Windows Settings → Devices → Typing.
Install Missing Dictionaries
When you saw the warning icon on your status bar like following, it properly means that extra components are required to download in order to support spell check for target language.
You could click on the “warning“ icon and follow the instructions to download & install missing dictionary files.
Windows 8/10
Typora uses the system spellcheck on Windows ≥ 8 by default, but you could still choose to use the built-in spellcheck module in Typora, instead of install language support for whole system, just click the pulldown button, and select “Install dictionary file only for Typora“.
Install Language Support on Windows 8/10
You could install the language using the regional settings:
Once the additional language is added, Typora will need to be restarted.
Install Language Support only for Typora
Click “Install dictionary file only for Typora“ from the pulldown button when Typora displays the “dictionary missing“ notification, Typora will automatically download needed files and use “hunspell“ module for spell check. You could learn more in following section.
Linux and older Windows
Click “Install language Support“ when error notification of spellcheck is shown, Typora will display the license info of the dictionary file, and then after click “download“ button, Typora will download the dictionary file. You could see the download progress from the status bar.
You could download needed language manually from https://dict.typora.io/, and put them under folder {typora-user-folder}\dictionaries, you could find the location of {typora-user-folder} by opening “theme folder“ from preferences panels, and then open its parent folder. On Linux it looks like ~/.config/Typora, and on Windows it looks like C:/user/{your-user-name}/appData/Roaming/Typora.
License info
Source and license info of those dictionaries are in https://github.com/typora/dictionaries.
Learn and Unlearn Misspelled Words
You could tell Typora to learn a new word by select the word and click “learn spelling“ from context menu. To unlearned a previous added word, just select the word and select “unlearn spelling“ from context menu.
Nota Bene: There’s no spell check for Chinese/Japanese, when those languages are selected, Typora will use en-US spellchecker to check ASCII words.