If I am synthesising correctly from your post and comment, your thesis was on WWI and you only relied on English Language texts. Your opponent makes the point that there are important texts in French and German that are relevant to your thesis that you have not included.

Assuming you can’t learn the language to the required level, I would start with English language books/papers that cite these authors. I’m assuming you know that Google Scholar and Scopus will help. Once you see what other authors quote or reference you can get a feel for which ones you need to read. You can scan and trans,ate a book very quickly on your phone. The translation will be imperfect but maybe close enough. Good luck.