Nothing would give me greater pleasure than to say 'extinct member of Euarchontoglires', except that I've tried it unsuccessfully now three different times, and I think I may have sprained my tongue. So I hope you don't mind if I just call you Pro.
Japanese has three alphabets (well, Kanji only kinda an alphabet but that's beside the point). Hiragana is for words that were originially Japanese, kanji is for words borrowed from Chinese symbols and katakana is for words borrowed from other languages. For example, the words lettuce and potato. And the Japanese word for bread is "pan" from the French, and the word for a part time job is "beito" which comes from arbeit the German word for work.
So, what I'm trying to say in my round about way is yes, they are just Japanesizations. But no, it's not unusual and it's nothing to do with the Japanese diet. :)
Did anything I just say make any sense, or help at all?
Correct me if I'm wrong, but retasu and poteto are just transliterations of lettuce and potato in Japanese, right? Isn't it weird? Maybe it has something to do with the fact that Japanese don't consider "western" greens as part of their diet (again, correct me if I'm wrong).