That depends on the target tool for those translations.
The file formats for translation files for MacOSX/iOS (.strings files), for Android (strings.xml files), for Linux (.po files), and a multitude of other development environments, are all different.
If one was to provide a tool for translators to work on this kind of translations, what one could do is to keep the translations in a database (original string/language, translated string/language), and have little tools to convert the various file format into and out of the database.
Using a database would help recalling common strings.
Now, in translating interfaces, not only strings need to be translated. If only, for some interface definitions take human input to adjust widget size according to the text (which is of different size in the different language), but also because translating involves more than just the text. Right-to-left script systems may need different layouts than left-to-right script systems. Icons may need to change according to the culture relative to the language. (And data may need to be computed or formatted differently according to the language, but this is for the developers to care about).
Therefore beyond the translation of strings, a UI translator will need to master the development tools used to edit the interface layouts (Xcode on MacOSX or iOS, eg. eclipse on Android, etc), to be able to provide a comprehensive translation service.
Perhaps considering some professional tool already developed would be a good idea? http://kilgray.com/webshop/