TeXShop is showing a spelling error (red dashed underline) for almost every LaTeX command. There must be a way to disable this so I can only check the spelling of my content.
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Searching on Google I found this.
Edited: based on Sean's comment below I decided to insert here some caps from the original website to avoid broken links.
Note that it is possible that aspelle fail to compile dictionaries in Yosemite or El-Capitan. Then this link can be helpful:
Specially it is possible that you did not accept license agreement for Xcode which can be done via terminal and
sudo make --version
and entering the password when prompted, pushing space bar to pass through several pages and finally typing
agree and hit enter.