The language selector on Special:NewItem only matches language codes when suggesting languages. Entering a language name like "German" has no effect.
Also submitting the field with language names, instead of language codes, yields The supplied language code was not recognized..
This is especially relevant as the language codes aren't translated (or even transcribed), but the language names are. Due to this, users need to use the (latin) language code to select their language, or scroll through the whole list of languages.
This is probably the case ever since this was introduced in 989958ac50ab9cac2de1d34bbd54a7050cb8b9c7.