VOX uses system settings for interface language as most of the other apps.
iPhone Language has the first priority, then the list of Preferred Languages used, the application uses the first supported language in this list.
You can find this list in Settings - General - Language & Region.
VOX supports English, French, Japanese, Korean, Russian, Simplified Chinese and Ukrainian.
So, to force VOX to use English, you should ensure that in Preferred Languages Order list English is placed higher than other supported languages.
We are looking forward to adding Traditional Chinese and reviewing current Simplified Chinese localization of VOX app. If you know someone, who would like to offer VOX team some assistance with the mentioned languages and localizations` reviews, please contact let us know, we would greatly appreciate that! Have a nice day\night!