I read that children's book somewhere between 2004 and 2011(ish), in French, though I wouldn't know whether it was a translation or not. I don't know for sure when it would have been published, perhaps in the 90s-2000s. If memory serves, the cover was blue-ish, with a drawing of a smiling young girl in knight armor, riding a horse, perhaps with a couple additional characters. The book had some illustrations, at least a drawing of green peas in their pods, and the main character in armor, holding a spear. I wouldn't be surprised if it had been published/translated by one of the main French children's magazines, like J'aime lire.
I'm about 90% sure the main character is a girl, about 10 years old, let's call her Sasha. Sasha is transported back in time, I think when reading a book about knights in her bedroom and wishing she could see the era, but that may be influenced by memories of Magic Tree House (which it definitely isn't). Sasha ends up in medieval Western Europe, though I don't think it was an Arthurian setting.
Throughout the story, Sasha meets the son of someone important (nobility?), but whom I also seem to remember being a squire. The boy, let's call him Bob, is about her age, perhaps a bit younger. His uncle travels a lot, and at some point he's showing Sasha the "newest" vegetables his uncle brought from America. The French quote went like this:
– Ce sont des légumes nouveaux, dit fièrement Bob. Mon oncle les a ramenés d'Amérique.
Sasha retient un rire. Il y a bien longtemps qu'elle connait les petits pois et les haricots verts !
Which I'd translate to:
"These are new species of vegetables," Bob says proudly. "My uncle brought them back from America."
Sasha holds off a laugh. She has known green peas and beans for a while now!
... and something about not wanting to hurt Bob's feelings, as he seems very proud to be able to showcase these veggies.
Bob shows her around, and she eventually takes part in a joust, which I guess she wins.
I have no memory how, but I remember she makes it back to her own time.
I've tried Googling, in both French and English:
livre jeunesse voyage temps chevaliers "légumes nouveaux"(
children's book time travel knights "new vegetables"), along with several variations of the quote (
"légumes nouveaux * oncle"), and browsing exclusively on Google Books
livre jeunesse voyage temps fille chevalier écuyer joute(
children's book time travel girl knight squire joust)
- as well as filtering the Google Images results for variations of these keywords by the color blue, again without success.