>покупи" (russian) -- this is incorrect. There is no such word in Russian. Kypi is a word, but I doubt it's used in the forms. May be the whole 'Purchase' example is not very good, as I don't remember having button saying Purchase in English forms either.
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>I am thinking we will use present indefinite form in Russian, so it will be 'Kupit' '
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>"покупитъ". -- no such word in Russian, correct one is
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>"купитъ".

I guessed from "покупателъ", which may be a word I imagined... my Russian is best left read-only. But I figured if I made a mistake, some of the resident speakers would jump in :).