"If it is nice ..." is perfectly natural English (and is the direct equivalent of "s'il fait beau..." since neither explicitly mentions "weather/le temps"), but this is currently rejected.
Why is, If it is nice we are planning to eat lunch outdoors, not correct?
It was an oversight. This and other reasonable variations have been added.
Why doesn't "look forward to. . ." work as a translation of "prevoir"?