有人坐左我地個位, 英文點講

It should be "someone has taken our seats" .

You should not use past tense because they are still occupying your seats. If the action happens in the past and continues until the moment of speaking, you should use present perfect tense ---> has taken.
Hope it helps!

有人坐左我地個位, 英文點講
No.2 - Someone took our seats (best)
or more officially:
Somebody had occupied our seats.

Someone sat on ours.

someone took our seats
or someone sat on our seats

坐咗某人個位嘅短語: take somebody's seat
so你個句子如果表達有人坐咗你哋個位應該用:
Someone (They) have taken our seats.
or: Our seats were taken./ Our seats were occupied