As of iGrafx 2009, sheets support functions similar to those used in typical spreadsheet applications such as Excel. However, these functions are supported in English only. For example, the English function SUM works, but other language versions of the function (e.g. SUMA, SUMME) do not work when typed in directly by you.
However, a copy-paste action from your spreadsheet application that uses a non-English language, into a basic sheet, works fine; the function is translated into English. So you may find it more comfortable to use the function in your mother language, and simply prepare the sheet in an application that supports your language and then copy/paste it into iGrafx.