Are you talking about humans that are alive? If so I call BS. If you pulled something thru when attempting to evoke an alive human, I can pretty much guarantee that it wasn’t what you thought it was.
Now if you’re talking about a human that has passed, you don’t even need to wait for a personal seal or anything. Most humans weren’t practitioners of magic and aren’t aware of what seals and sigils even are. Also, you can’t just make someone’s spirit talk in a foreign language. Keep in mind that spirits living in the in between are essentially a snapshot of who they were at the time of their passing. That means their name, their habits, memories, pains, everything is still with them. That includes their native language. I can’t call on Jane Doe from California who’s spoken English her whole life and expect her to speak a foreign language, let alone Sumerian

if that “human” spirit talked to you in Sumerian I would be very careful what it wasn’t another spirit pretending to be whatever you thought it was. Now, if the spirit was Sumerian that’s a different story but we’re talking about a spirit that’s been roaming around for thousands of years?
What threw me off is your wording of “I always MAKE the spirits communicate in a foreign language.” Like, if you can understand Sumerian, major props to you. But you can’t make a human spirit talk a language that they didn’t know. I was actually just talking about this with my godfather yesterday.
All you need to do is find someone who died tragically (because this makes them more prone to still be hanging around here) not too long ago. Preferably no more than 10 years ago. They should be in their 20-30s, again because younger people are more likely to be pissed that they died young and refuse to leave the physical plane. Use their name and picture, invoke your protectors and guides, like a white candle with water, some coffee, a lit cigar and call them up. Use a pendulum to see if they wanna talk or if they’re even around. It’ll be evident by the lack of movement from the pendulum or the lack of sensing fluids within the area.