In general, language-dependent texts of SAPUI5 apps are stored in properties files (resource bundles):
There's a default properties file for the source texts which contains all externalized strings from the app. It looks like this:
<name>.properties (<name> is an identifier)
An app can have n properties files.
During the translation process, properties files are created for every language the app is translated to and they look like this:
<name of property file>_<locale>.properties (<locale> contains the language and an optional country code)
Example: i18n_en_US.properties contains the American English texts of the i18n.properties source file.
This is the recommended approach for translating texts of SAPUI5 apps, for example SAP Fiori apps.