In the Premium account, apart from the "Default Language Set" where you can keep the documents in your main language, you can create up to 3 extra data set.
Each data set has a limit of 18 resumes and 18 cover letters. Meaning that you are able to create up to 72 different resumes and cover letters (the documents can be created only for yourself, as the accounts are for personal use only).
When creating a new data set, the documents will need to be created from scratch as there is no synchronization between the data sets. All the headers and fields are editable, so you can easily start writing and editing the documents in the 2nd data set in your 2nd language, and they will not affect the ones in the other data sets.