First and foremost, you should ask forgiveness from Allah SWT in every slip, mishap and even when doing good. It is the habit of the prophets including Nabi SAW that they sought the forgiveness of Allah even after a good action. This is why we are recommended to say "Astaghfirallah" after Salah/Namaaz because even though we offered our prayer, we did not do so as Allah deserved, i.e. with full concentration/devotion.
A person who asks for forgiveness with sincerity is likened by Allah than another individual who doesn't sin at all and does not ask for forgiveness. So always ask for forgiveness, regardless of anything.
Secondly, there is certainly a difference of opinion from the scholars as to what the consequence is of eating haraam.
One example I heard from a scholar was the likeness of driving towards a ditch in a road but not knowing that the ditch is there. You're still going to fall in regardless if you know of it's existence. Haraam food is exceptionally bad and whether you know or not, there is still an effect on your Imaan.
If your case, your are still at fault. If the food is labelled in a different language and you do not understand then it's very, very important that you abstain from it as you are unsure. The fact that your brother saw the word Alcohol on the packet can only mean you did not undergo enough investigation.
...But this is where the question lies. How much investigation do you perform? A lot of my friends eat Vegetarian meals in takeaways which do not have Halal meat. The problem here is mix-contamination - The chef might use the same cutlery/pots/pans to cook the meat with the vegetarian food. But how are my friends to truly know if they are using separate tools for Veg and none Veg meals?
Taqwa. It's the Taqwa, the fear of Allah and our level of Imaan which we have that will help us abstain from something.
I won't comment on whether or not your prayers will be accepted as I don't know enough but the above will hopefully guide you in to doing the right thing in the future.