66 is seks-og-tres, i.e. seks (6) og (and/+) tres (60). The latter is technically "tresindstyve" (which is similar to the French, 3x20), but I can guarantee you that the number of people who actually know and care about that is infinitesimal. Additionally, "tres" is not really a word that can be mistaken for another number word. What usually screws people over is pronunciation and thus "tredive" vs "tretten" which sound shockingly similar in Danish.

(This happens a lot in Danish. It's incredibly sloppy on pronunciation and people somehow mostly still understand each other.)

EDIT: I had to think about it for a bit, but the situation for 77 is actually equivalent. No speaker of Danish actually thinks about "halvfjerds" as a compound number -- it just "70". I suspect it's actually very similar for French speakers, but their hyphens do make it harder to just lump the thing into a single concept/number :).