How can I change the TeXShop dictionary to American English?! It is set to British English! It might be the whole mac's dictionary that is being used, I am not sure. In that case, how can I change it in the system?!
The default language is the one chosen in the “Language & Text” System Preferences pane, where you can set the region to “British”. Or you can choose the language in the TeXShop Edit menu (“Show Spelling and Grammar”, that in the picture becomes “Hide”)
Go "Textshop" => "Services" => "Services Preferences" => "Text" Inside "Text" page, click "spelling" on the right.