Powers are, by definition, stronger than stunts, by (arguably) at least a factor of two.
I'd still tend to restrict such a power a bit - my version would be something like, "If somebody talks to you, in person, using language X, you can respond using language X." - so no deciphering ancient runes, no running through dozens of languages fishing for the one your untalkative prisoner will respond to, no talking russian to some guy over a telephone, etc.
It could also depend on how often it's going to come up - if the GM knows they plan a number of plot points around dead languages, the ability to understand those might be worth -2. For other games, where most people speak english and other languages won't come up much, even the ability to speak, read, and understand all languages could easily be -1 as a power.