Because your conditions (Are you using Hungarian-localized Windows? , What are your current Windows' Regional Settings? , How did you get the problem when it "does not work"? How did your Greek string appear on the screen?, etc.) are not clear, it is difficult to figure out. Anyway, Did you try to specify the "Symbol" font directly such as "\f:Symbol(your string)"? Does it work?
Thank You very much, it works that way! I don't understand, why \g() is different on my system. I use the default settings for tools/options/Text Fonts Greek -> Symbol. Is it a program fault under XP Pro?
P.S: I have XP Pro English, with Hungarian local settings. The problem was that \g(m) appeared as (bit smaller) m and not mju, as it should have been.