Everything on the screen (the "user interface") in ContractorTools is in English, but you can customize the documents for your customers to use a different language by using "Custom Labels".
To set your Custom Labels, select "Company Settings" in the main menu, then select "Custom Labels".
The Custom Labels screen allows you to specify all of the words that will appear in documents that you give to your customers. The English word appears on the left side of the list and the current word specified appears on the right.
ContractorTools ships with all of the labels set to English, so initially, all of the labels are the same as the words in English.
To change a label, tap on it, delete the word in English, and enter the word that you would like to replace it with.