Most commonly if is translated as wenn.
Historically/Semantically wenn has a temporal component (as in the English when and the German wann), while falls is more straight-forward conditional.
In the spoken language and informal written language the use of the conditional wenn is widely accepted.
The conditional wenn and falls also seem to have slightly different connotations regarding certainty and/or desirability of a future event. During my time as university student in German Linguistics my student group and I wrote a paper on that after having interviewed 200 students. We came to the conclusion that a significant (over 85%) amount of the students used falls when talking about a possible event in the future that later was described as being undesired, while wenn was used to describe neutral or positive possibilities. Also, in slightly different context with all positive possibilities, wenn was used to refer to future events where the student was leaning towards "it's likely that this will happen", while falls was used in cases where the student thought the event to be unlikely to occur.
Please note that this was just a simple research project for a paper. The tendencies we found were significant within our data, but the sample data was rather small and might have some unaccounted local bias to it – it's an interesting observation that fit's well my with personal experience, but it's not representative.