i18n: make sure 'en' in Accept-Language is recognized as having 100% coverage

The workaround in 7c7d6b5c07c7 no longer works after upstream addressed the
main issue and released the changes in TurboGears 2.4.3 .

Setting `i18n.native = en` in the .ini works as a workaround.

The native language for translations is an implementation detail that users
shouldn't have to configure, so we define it as a default value without making
it explicit in the generated .ini template files.

Note that even though TG will figure out that languages like `en_US` should
fall back to using the `en` `kallithea.mo`, it doesn't consider `en_US` native
if `en` is in the native list but `en_US` isn't. We thus include the most
common aliases for `en` in the list.