Your Subscribers can set the preferred language for their News Digests by going through the personalization process.
Subscribers have two ways of accessing the personalization process to select/change their News Digest language.
When a Subscriber receives the Welcome Email, they can go through the personalization process and select the preferred language using the drop-down list under "Deliver all my News Digests in" inside the Your Format tab.
News Digest Footer:
In the footer of previous News Digests, Subscriber can click the "Personalize this email! Schedule, stories, format" link to open the personalization process. Then, Subscribers can edit the selected language using the drop-down list under "Select Language" inside the Delivery tab.
Google will automatically translate the Subscriber's News Digests into the selected language, and Subscribers will also have the option to translate the News Digest to English (or another language available).
The image below displays all the languages available for your News Digests to be translated into.