I've done a lot of googling and it seems quite a lot of people have been complaining about the UI fonts in Google Earth and Picasa. I've read quite a number of imaginative explanations but I haven't found a fix that would work on my box.
Nevertheless, it seems that the newer versions of Google Earth for Linux and Picasa for Linux aren't real Linux applications: they use Wine libraries. And it's also possible that the Wine libraries don't work very well with Nvidia drivers.
So I'm waiting for the next versions.