Reduce the inconsistent use of "language" vs. "lingo"

Now we’re just using the word “lingo” for the macro defined in
PageTemplate (where the abbreviation is useful), and using the full
word “language” everywhere else.

It’s possible that we should be using “locale” instead of “language”,
what do you think?
Get rid of the separate language prompt tiddler

More consistent to stick with .tids files
Add note about joining Google Groups without a Google account
Get the remaining core control panel tabs prepared for translation

We’ll do ThemeTweaks separately as it’s part of the Vanilla theme
Add a demo of alerts and tweak styles
Making more things translateable

Now we’re done with the text that lives in tiddlers, and we’ll need to
get on with the text that is embedded in code modules.
Make the control panel "internal" tab translatable
Fix typo
Add missing field descriptions
Rejig the control panel to use nested tabs for Appearance and Advanced
Make type information translateable
Put language files in alphabetical order
Fix view widget so that it works with indexes
First pass at switchable colour palettes

Fixes #410

Still some cleaning up to do, and we need a way for incomplete palettes
to inherit from a base palette.