There are translated strings associated with simulation repositories and common-code (library) repositories. Strings for the English locale ("en") live at the top-level of the repository, for example: geometric-optics-strings_en.json, scenery-phet-strings_en.json.
After making changes to the English strings file, note that you need to run "grunt modulify" in the repository to regenerate the js/*String.ts file for the repository (e.g. geometric-optics/js/geometricOpticsStrings.ts ,scenery-phet/js/sceneryPhetStrings.js). These *Strings.ts files are how you import strings into code, for example:
import geometricOpticsStrings from '../../geometricOpticsStrings.js';
const helloWorldNode = new Text( geometricOpticsStrings.hellowWorld, ... );
Strings for other locales lives in the babel repository, and have a json format that is identical to the English files. These files are not typically created directly by developers. They are created by translators, using PhET's Rosetta translation tool - more info at https://phet.colorado.edu/en/for-translators
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