This phrase is a combination of the numeral one (ān) and a modifier meaning “all of a set” (tēna). Together they denote a complete set of one. In the first sentence of the Babel text:
iēlte la anmārwi pa antaxōni ān tēna;
ān tēna modifies yesterday’s word antaxōni “language” to make a phrase meaning a set of only one language. So the sentence can be translated as:
Once long ago, the world had only one language.