Fonts gone??!!

I can realize if far-eastern fonts are not installed then those asian characters wouldn't be able to be shown correctly, but how to explain if the English interface of a software disappear??!! This exactly happens in front of my eyes!! This trouble happens on Linux-Opera, and all the interface which are supposed to be English characters are now all blank, and even the English websites also show nothing! Odd enough, it can show asian websites without any problems! Now I'm using FreeBSD native Opera to write this diary, but without Linux-Opera it's unconvenient to browse those Flash-powered websites... Need to find a way to go...