Make documents bilingual

One-sentence description

Make it so documents can be uploaded in both Dutch and English, and display one according to the user's language preference

Desired behaviour

Since the association will gradually produce more documents in both Dutch and English, they should get a place on the website. In my opinion, the best way to do it is to have two fields for each document to upload one of both languages. Then, upon loading the documents page, the documents in the right language are loaded. If the corresponding language is not available for a certain document, it could fall back to the other language.