Also added feedback to source & target language menus to indicate which
languages are supported by the selected translation method. For Apertium, the
source language menu shows all possible source languages, and the target menu
shows the possible target languages for the chosen source language.
Note though that the unlexer does the wrong thing with initial words that are
supposed to be capitalized, e.g. "I am ready.", "Spanish wine is good.", so
these sentenses are not translated at the moment.