This reminds me of something that I learned in a software localization class ages ago. The class advised that software developers should allow twice as much screen space for short bits of text (such as dialog box button labels), and 30% extra space for bulk text (a paragraph or more), because English is much more compact than many of the other languages that software is published in.
If the French-language text of Mongoose Traveller is 30% larger than the English-language text, I can see how that could make layout a significant task.
Also, I would imagine that some illustrations would need to be changed if they have any English-language text in them. Localization is not just a matter of translating ordinary text; diagrams and art need changes too.