Video language should be detected and set by default on recordings. I currently have to manually set that on every recording to get captions generated. If ambiguous (e.g. English and Spanish detected), use my O365 language setting to resolve the ambiguity. If I manually change the language, honor my change.
Note: There are Office 365 and Stream settings for language, but recordings don't honor those settings. I considered asking that they be honored, but it would clearly be more user friendly to just detect the language used, especially given that you only support English and Spanish right now. This might become more difficult eventually, when more languages are supported, hence the suggestion to use our Office 365 language setting when auto-detection is ambiguous.