locales mapping for language fallback: do not repeat language

currently the language fallback is defined like this

locales:
en: [en, de]
de: [de, en]

the first option must be the requested language. this is counterintuitive and error prone. saying "en: [de]" would make english always only try german. we should add the locale itself as first locale i think, i see no use case of defining fallbacks where the requested locale is not the one to be preferred.