Hi Benjamin,
As you can see, the form will contain some inputs such as locale related issues like numeric input(comma decimal) and calendar display. We matched the form locale with the application locale to provide users a more consistent application usage experience.
You would expect that most of users in Brazil would like to read the menu such as submitting the form and browsing app's setting in their own language while they use Survey123 field app to fill out the survey. Imagine you are a native speaker of Arabic and can't read English at all, but you would need to fill out the survey form sending from other country which set English as default language. You would expect to be able to use the survey field app in Arabic to set some of the configuration and also be able to read any pop up messages asking you to take the next action. If you can't read "Yes" or "No" button, you won't be able to complete the survey easily if you can't read English at all.
For survey designer, you might find out a bit inconvenient to switch back and forth by toggling the languages but your users will appreciate if they can stay in their own langauge from the begining to the end of the survey filling.
Hope this helps,
Thanks,