I would like to say thank you to the forum, and especially to Sm1smom!
Recently, we received green cards, and the smallest thing I can give back is the interview experience.
The IO was professional and nice. My spouse (main applicant) doesn't speak English fluently, and we decided to take an interpreter with us since we were not sure if I was allowed to be an interpreter during the interview for the spouse (in all the forms like I-485, we mentioned that I'm an interpreter). You can take your friend or hire someone, but this person should be a legal resident of the US. The IO asked if we had the G-1256 form (this form is used when you use interpreter services). We had this form with us, and we provided it to the IO, and the form was signed and verified by the IO. After the oath, the interview was pretty standard – only questions from the I-485. The longest part was to verify original documents (school, university diplomas, marriage certificates, and so on). We had a denial of a visa once (article 214b), and I recommend that you not hide this information if you have had some issues in the past because the IO has access to your files. The IO asked if we had an apartment rental contract with us, but we didn't have it, and they said it was fine.
In the end, the IO asked if we had any questions, and I asked about the decision. The officer said that they didn't have any obvious reasons to deny the case, but we should wait some time (I don't remember the number of days) till the final decision because they needed to verify some information. The same day, in the evening, our status changed to GC was ordered.