First, the expression "außerordentlicher Professor" is no longer in current widespread use in Germany (although still understood; the exception is, as so often, Bavaria); a non-ordinary professor position is now usually referred to as "W2" (after the pay scale) or, in some states, "W3 ohne Leitungsfunktion". Most people who now hold such a position would translate it as "associate professor", although there are a number of significant differences between the positions that make this translation misleading in some regards.
It is also a very different position from the one Emmy Noether held (which was unpaid, for a start). In fact, Emmy Noether's position was actually that of an "außerplanmäßiger Professor", a title given to tenured university assistants after their habilitation, which entails most of the academic, but very few of the administrative, rights of a university professor. (The "nicht beamtet" previously disambiguated the two positions; the title was renamed during the Nazi regime.) This position -- which no longer exists in the German system, and, confusingly, was called "außerordentlicher Professor" (and now "assoziierter Professor") in the Austrian system -- has no analogue outside the very hierarchical traditional German system. Hence there is no canonical translation; if pushed, I would translate it either as "tenured research assistant" or "lecturer" (if there is no danger of confusion with the UK title, which would give the wrong idea), both capturing (different) aspects of the position.
Especially in a historical work, I would therefore do exactly what the English wikipedia article on Emmy Noether does -- use the original German title and give a parenthetical explanation:
the title of nicht beamteter ausserordentlicher Professor (an untenured professor
with limited internal administrative rights and functions)