I would rule that gibberish does not prevent spellcasting in most cases (my reasoning being that it affects the perception of listeners, rather than changing what is actually said). However, if the spell requires the target to understand spoken instructions (such as command or suggestion) then gibberish would prevent it from functioning.

ETA: To make my reasoning more explicit, tongues causes listeners to hear the speaker in their own language. It is my feeling that this is a change in their perception of what is said; the spellcaster is not actually speaking in (possibly) multiple languages at the same time. I would argue, then, that the reverse of tongues works the same way: the spell target speaks normally (including spellcasting) but is perceived as talking gibberish.

Last edited by Blackadder23 (11/27/2017 2:12 pm)

Rereading the description of tongues in AS&SH leads me to believe that Jeff does indeed intend for the spell (and it's reverse) to affect the actual speech of the subject or subjects, not just the perception of it. That being the case, I now conclude that spellcasting (and the use of magic items with command words) would be prevented (at least according to the letter of the rules).
 

Last edited by Blackadder23 (11/27/2017 8:54 pm)

I would probably not allow Gibberish to disrupt spells.  I see spells as vocal "work". Manipulating the energies to bring about an end result. The "language" of magic is for this work rather than communication.

Take for example the new spell "Dances" and its reversal "Square". The former allows the caster to do any dance for the purpose of dancing. Square would prevent those within 50' from being able to do any dance in a coordinated fashion, thus being me in middle school. Square would not prevent someone within 50' from moving a wheel barrow as this is rhythmic movement doing work not dancing.

Just my 2 pennies as I don't think this was the intention but perhaps an unintended consequence of the spell