There is a game answer and there is a social answer:
- You can make any choice that doesn't violate continuity.
- RPGs are collaborative fun, and you should involve the DM in the decision because that's what RPGs are about.
This distinction is implicit in your question as to whether you should pretend like you knew it all along, which could mean pretend your character knew it all along (not violating continuity) vs. pretend you, the player, would have made this choice at the time and just forgot to write it down.
You can make any choice that doesn't violate continuity
Your character has a class feature that requires a choice. As a new player, you neglected to make that choice. But even with experienced players, I have been in groups where characters are being created at the last minute, or even as the session is being played. You still get to make the choices that are legal to your character. You certainly don't lose a class feature because the choice was not made in a timely manner.
If you wanted to try to be completely honest with yourself about it, you could make a decision that was "what choice would I have made with the knowledge I had at the time", and even outside of RPGs I often find myself or other players making these kinds of decisions in order to keep a game going. For example, in a card game, a player drops a card showing it, but another player decides to make the play they would have made not knowing the exposed card. However, as a beginning player, you probably didn't have a lot to go on in making that choice, so any choice is reasonable. If you feel your choice would have been nothing more than a mental coin flip, you could choose to flip coin.
Finally, there are two reasons why any choice in this case is acceptable.
- This is an extremely minor ability.
- It's goblins today, but it could be orcs tomorrow, and eventually it will be ghouls or dragons.
Furthermore, while I have stressed that any choice that doesn't violate continuity is acceptable, it is not uncommon for DMs to allow retconning of choices. Adventurer's League allows character rebuilds up to Level 4 (though not in the middle of a session). And I have seen DMs retcon abilities when it becomes necessary to advance the story line. Of course, this requires consulting with the DM.
RPGs are collaborative fun, and you should involve the DM in the decision because that's what RPGs are about
The above addresses what the rules require, and the fairly low-impact nature of the choice at hand. I don't think it requires DM involvement. Consulting with the DM may nonetheless be a good idea.
Before I suggest why I think consulting with the DM may be a good idea, I'd like to address why I don't think it is required. I believe that any reasonable DM would essentially say what I said above, and therefore it is fair to make the choice that a reasonable DM would approve. If the DM were to say you missed your chance to learn the language, that would be unreasonable (and, frankly, not in the rules). Forcing you to make an undesirable choice (that does not violate continuity) would be unreasonable, e.g. if the DM's reasoning is "Well, since you now know you're fighting goblins, you have to choose orcs." Even requiring a coin flip would be borderline unreasonable, since I think a good DM would make allowances for a new player, while letting you know "It's goblins today, but it could be orcs tomorrow."
That said, it is not a bad idea to consult with your DM, for three reasons.
- It will build trust with the DM.
- You don't want to feel you are deceiving the DM. If you are this concerned about fairness, and choose at this point not to consult with the DM, you may also find that you feel deceptive about this choice. No player should feel like they are hiding what is usually out-in-the-open information, like character creation decisions, from their DM (or other players), and you should avoid trapping yourself in this kind of feeling.
- It will be an early clue as to what kind of DM you are playing with. I expect a good DM to basically agree with what I have said above, but not all DMs are good DMs. This site has a tag devoted to problem-gms. If your DM tries to make you feel stupid for missing the choice at character creation, or tries to "punish" you with an undesirable choice, I would like to tell you now, from experienced player (and DM) to new player, that that is not acceptable behavior.
There are (primarily social) benefits to involving the DM in the decision, but the correct answer, which I expect a reasonable DM to agree with, is that you can make any choice that doesn't violate continuity.