Still pretty kludgy, but workable. First you need to find out the name of the field for which you want to provide text. Look in the texts.properties file. There, for example, you might see:

fullscreen=Fullscreen

Now head for Tiger > Advanced > Custom Keys, and enter this:

fullscreen=Go Big!!

Now, when you make the album, the fullscreen button will show "Go Big!!" as the tooltip.

Yes, that's a much more difficult thing to handle. The jAlbum core doesn't provide a way to handle multiple comments for an image (one set in English, and one set auf Deutsch, zB), and having the skin handle it on its own get very messy, very fast.