We've added the ability to retrieve the translations for a specific language when using the Get Phrases endpoint of the Localize REST API.
Previously, translations for all languages were returned (which is still an option).
Simply provide the language code for your desired language in the languageCode parameter. With the language code option, you can target a specific language's translations, minimizing the amount of data that's returned in the call.
You can find the language codes here in your dashboard.