If your sister is a US citizen, she would probably be better off avoiding the tax implications that come with buying mutual funds that are not registered in the US. Teach her how to make a ladder of individual bonds instead.
Added: Actually, while individual local bonds may be the lowest cost solution for her, the tax paperwork is not completely painless there either, since she will need to keep track of the exchange rates at which the bonds were bought and sold or matured in order to calculate the capital gains or losses in US dollar terms for the IRS (even if there were none from her perspective in her currency). This problem is avoided if she can get CDs there instead, for which she would only pay taxes on the interest.
Further added: I just noticed that you specified Euro-denominated bonds, not koruna. (I see that Google sez: CR will adopt the Euro in 2012 (maybe).) In that case, other possibilities are FXE and Everbank (if they will take her -- they may not).