Doubt: in Italian we usually say, as in French, "limoni, grazie" (des citrons, merci), but my English teachers always thaught me that in English they don't use "thank you" but "please" (for example Q: "tea or coffee?" A: "tea, please" in Italian is "té, grazie"). So, even if literally speaking thank you translates merci, in this case shouldn't be used "please"?
Sometimes "des" is a contraction of "de + les" meaning "of the". When "des" is used as a partitive article as it is here, it may be translated as "some" but in English it is often omitted. I.e., Je mange des citrons = I am eating lemons (or) I am eating some lemons. http://french.about.com/od/grammar/a/articles_4.htm
but it could never be "de" for singular in this case, only "du" - but even that only for partitive article (non-countable noun, which lemon is not in any case, as it seems to me). Here you have indefinite plural article "des" - not partitive plural article "des". Hope this will help.
Nine times out of 10 English uses "please" here. If lemons were thrust upon you, I'd expect to say "thank you!", but the punctuation in the question does not support that interpretation. I would have expected, "Lemons! Thank you!" Or "Lemons? Thank you!" the question looks like a response to "What would you like?" and that requires, "Lemons, please!" (or a simple full stop).
The psychology in English, I think, is that you say thank you when something is in your hands, and please before you have it.