I'm not sure that it is a given that the numbering needs to be different from the original footnote numbers. Footnotes numbers are not necessarily a canonical part of the text, and since it would be virtually impossible to ensure that the pagination of the translated work would be the same as that of the original work, the numbers of the original footnotes will sometimes vary anyway.
My primary concern here would be with the ergonomics of the thing. Having two independent set of footnote numbers will be difficult for readers to navigate and understand. Lacking some compelling reason to treat the source footnote numbers as canonical (which will require preserving canonical pagination as well) I would use a single set of footnote numbers and mark the translators notes as such.
If you want to distinguish them more clearly, I would suggest some typographic convention that preserves one numbering sequence but perhaps makes the translator's notes and their numbers in the text bold or italic.
It that does not satisfy, I would suggest putting translator's notes in the margin next to line in question, so that there is no confusion with the footnotes. (This would also work in those cases were you might want to place a translator's note on a footnote.