Content localization use default locale when unsupported
Serve localized content in the site's default locale to users whose preferred language is not listed in 'content localization supported locales'.
But mine is fi. The reality™ (meaning: I believe so) is if a visitor is using something else than what I has enabled, that visitor isn’t a Finn or can’t read Finnish, but now is Finnish content served. I would prefer English in that situation, but because that forum is heavily Finnish, I don’t want to change locale to some en variant.
If you want “en” to be used, then the site default locale would make sense to be “en”.
If you disable the setting, there won’t be a fallback and the user will just see the original language (unless it is translated to a supported language, per the description).
A Finnish visitor who has set their browser locale to “fi” (highest priority) will see “fi” (if they don’t, it’s a bug, let me know). I feel like you can go ahead and set your default locale to “en”.