Not entirely sure, it seems like the stringtable.xml can overwrite any of the standard values set in string.xml.
To answer the last question, it seems to be an empty stringtable in the /english folder, to indicate that it does not overwrite any values from string.xml This in turn leads me to believe that it does expect 1 stringtable.xml per language and thus that just putting a strings.xml file in a /language folder will not have the desired effect.
You can however test this quite easily, by just copying any existing file, translating the an easy to find message like "quit" or something, and then checking if it works or not. If it does you can then translate the rest in the same fashion.
Anyway, it seems to me like you'll either need to make a stringtable.xml in a language folder, or change the root strings.xml (though in that case, it will break the English support as it is). A quick test as mentioned above will give you a definitive answer.