Loki was genuinely and truly shaken. He had felt the call of the Tesseract and not even known that's what it was, and to heed it would have heralded disaster for them all. He wanted both to leave as fast as he could, and to remain to make sure that nobody did anything foolish.

He remained in the Aesir tongue. Even if it was rude to the mortals, it was important that he give this information to Thor first, too shaken to even reprimand him for knowing so little.

"Thanos, the Destroyer, this is his creation and something he seeks. It is far beyond the power of the Casket of Ancient Winters, and it does not belong on Midgard. It does not power their realm, it only endangers it, for if Thanos discovers where it is then he will destroy this whole planet to recover it."