English Translation
Why don’t we go in?
A better fit for this would be “What if we went inside?”
Because of the “-pä” and the “-isi”, this is a more of a polite suggestion than a hypothetical.
These are both idiomatic ways to politely suggest going iniside. They are rhetorical questions, and not literally asking “why?” or “what if?”. So it’s kind of moot to try and translate them literally.
“Why don’t we…”: You’re not really expecting a “Because, …” answer
“Mitäpä jos…”: You’re not really asking “what would happen if …”
And another example from my native language:
“Wie wär’s wenn wir reingehen?” - literally “How would it be if we went in?”
Again, not actually asking that question.
this is a more of a polite suggestion than a hypothetical
Exactly. That’s why you use some idiomatic equivalent, and don’t translate literally.
To my non-native but experience ear, “What if we went inside” is less idiomatic than “Why don’t we do inside?”