In the Settings (Library → Settings) you can change the language for transcripts and captions. You can use English (default), or pick one of the languages that are available in iOS. You can add additional languages in your iOS Settings in Settings / General / Language & Region.
Languages that support Dictation (speech to text) will be listed in Settings / General / Keyboards / Dictation Languages.
After you've added a new language, it can take a few hours before the language will be available on your device and unfortunately not all languages are available.
Please send us a quick note to [email protected] if your language is not supported, to help us prioritize the support for future languages.
Read more about Dictation languages in iOS in this Apple help article: