All the strings used in iTop GUI are located in dictionnary files. Those files are part of the datamodel, so a compilation is needed after each modification (run the setup again or use the toolkit).
The embedded dictionnaries are located in datamodel/2.X subdirectories. The files are named like this : <lang>.dict.<module>.php. For example : datamodels/2.x/itop-config-mgmt/fr.dict.itop-config-mgmt.php. Each file contain a PHP array, with the dict key as index and the translation as value.
You'll have to create a fork from iTop main repository, then do your modifications based on the develop branch, and then create a pull request.
A detailed procedure is available in the GitHub help pages : Creating a pull request from a fork.
The pull request will then be analyzed by iTop maintainers, and eventually merged in the main repository.