How can I set the gadget so that it will default as English to Spanish or Spanish to English which is what I use most? Every time I reopen the page, it defaults to English to German and I have to change it every single time.
The selection is saved in a cookie. When you allow cookies it should work.
In Firefox I go to Preferences under Menu/Add ons/Extensions and there are default From/To languages. Set these to auto/en or auto/es. I don't think you can give it a default target when translating from your default language.
I had the problem that it decided that my default target language was Spanish which I rarely read and don't understand. This did solve that problem.