For people living outside one of the native English-speaking countries, the language settings will typically be one of two alternatives:
X language (for the UI) and X regional settings (for date/time format, time zone, currency, distance, speed and whatnot)
English language (for the UI) and X regional settings (for date/time format, time zone, currency, distance, speed and whatnot)
Flow offers only option 1; you can choose X language and X regional settings or English language and English regional settings - these are the same; option 1.
This is pretty basic, but English language speakers never experience the problem, so a US company like Microsoft can be forgiven for getting it wrong - but you've been in this business quite a long time, so it's time to get it right 😋
BTW: Don't use the browser language settings - those are for web pages, i.e..documents. Office 365 is a collection of cloud-based applications, i.e. software.
Most non-English speakers that want English software still write mostly in their native languages, and may prefer to browse the web in their native language, so don't confuse these settings; content and applications should not be forced to the same language settings.