If an account is autocreated at Commons, the language preference for the user should be the one from the homewiki.
has "eo" as setting in the homewiki (eo_wiki), but the interface appears in English at Commons. With this enhancement it would appear in Esperanto.
This is different for Commons than from other wikis as Commons hosts files for other wikis. If the sample user would edit enwiki, the language should generally be in English.
Some related Commons VP discussion is at