I have a player in my game I'll call "Joe" running a Bard I'll call "Hansel."
Hansel has the Actor feat, and does not speak Orc. Hansel listened to an Orc for 1 minute a few adventures ago, a 2nd PC taught Hansel one short Orc phrase, and I allowed Hansel to repeat the new phrase, in Orc, mimicking the voice of the Orc Hansel had just listened to.
Joe has recently expanded this to insist that if the 2nd character now says several Orc phrases over the course of a minute, Hansel can repeat each phrase at will, mimicking the voice of that Orc. Joe believes that Hansel can also rearrange the words to create new phrases, on the fly in conversation with Orcs – and that this can also fool any Orc into thinking it is an Orc from their own War Band, so long as there is a door there to muffle the sound.
Joe claims that anything short of meeting this demand undermines the entire usefulness of the Actor feat, and therefore robs his character of 4 levels worth of adventuring.
Can the Actor feat allow a character to effectively speak a language they don't know?
I think Hansel could mimic a specific Orc's voice, after 60 sequential seconds of listening to it speak consistently (though not necessarily persistently), but only using languages Hansel already knows.
Best Answer
No, Of Course Not.
First, this is not what the feat says. It says you can mimic the speech of a person, or the sounds of a creature. It says absolutely nothing about understanding the speech as it is being spoken, much less performing the kind of complex adaptation involved in learning the language whole.
Second, this is just not how languages in the real world work. Real languages aren't just one to one mappings from this vocabulary to that vocabulary-- life and language would be much simpler if they were. Real languages differ by grammar (try learning Latin or Chinese as an English-speaker) and often by phoneme sets, patterns of inflection, idiomatic expressions, local dialects, etc. The most you'll get from a few minutes or even a few hours of exposure is a sort of an embarrassing pastiche of ways to ask where the bathroom is in very broken Orcish in a presumptively Human accent.
Third, this should not be a back door into learning infinite languages, effortlessly. Not only does the text of the feat not support this, but other rules give insight into the appropriate level of effort for learning a language:
The Linguist feat, on page 167 of the PHB, gives an additional three languages. Alternately, spending 250 days of downtime-- at the cost of 1 gp per day-- studying or learning a new language (PHB page 187). These are the levels of effort or investment required to learn a new language in a meaningful sense, and they are both much higher than "Odd side effect of a previously bought fully powered feat.)
In short, the feat (in the first point) says nothing about the ability to improvise speech which has not already been heard. In some cases that will be reasonable and in other cases it will not. The second point argues very strongly that it is reasonable to improvise speech in a known language, but not in an unknown language... and the Actor feat does not grant a new language on one minute exposure.
There are a range of concessions you might make, which are up to you as a game master:
You might allow the concession you've already made: Learn one phrase at a time. I'd be a little worried about setting a bad precedent (depending on the player) but I'd probably allow it.
You might allow the Actor to come up with a string of sounds that sounds like Orcish to someone who isn't a speaker. I'd probably allow that.
You might allow the Actor to deliver a set speech, with sufficient exposure to the speech and some practice; or perhaps a set of stock phrases. I'd probably allow that, too.
You might even allow a scene from Cyranorc de Bergero, where someone is whispering the right Orcish phrases into his ear and he says them... but you'd have to wonder why the extravagent ruse. And even there I would start levying heavy penalties to the necessary rolls.
Or, you can always say, "No, languages in my world are much simpler, so this all works." But you are not obligated to, and the player simply isn't empowered to. And if that is literally the only use he has for the Actor feat, I think this failure of imagination is on him.