First, let me say that as my nick says well, I'm not an airman. But I enjoyed enough this loooong thread to propose you something official about the translation of "ligne de vol" from Transport Canada, and from an officially bilingual country. From their airworthiness manual in French:
Rglement de l'aviation canadien - Partie V, Manuel de navigabilit Chapitre 525, Sous-chapitre C
scroll down the page to section 525.479. Do you see something familiar in the section's title? (and the first words of the section for that matter)
Now get back to the top of the page and click on the "English" tab. Get back down to 525.479 and now, what do you see?
OK, too lazy for this exercice? So, "ligne de vol" = "level attitude" at least to the ones making the flight rules around here.
Hoping this settles that one for good and for all.
Jean