Support new german languages added since `babel-german` v2.99

[muzimuzhi]

(Originally reported by @hseliger in cgnieder/xsim#136.)

In v2.99, babel-german deprecated languages austrian, naustrian, ngerman, nswissgerman and swissgerman in favour of

• german-at, german-de, german-ch (alias german-austria, german-germany, german-switzerland) for contemporary (1996) orthography
• german-at-1901, german-de-1901, german-ch-1901 (alias german-austria-1901, german-germany-1901, german-switzerland-1901) for traditional (1901) orthography

See CTAN announcement for babel-german v2.99 and texdoc babel-german for more info.