Albert Barnes Commentary Jeremiah 5:15

"Lo, I will bring a nation upon you from far, O house of Israel, saith Jehovah: it is a mighty nation, it is an ancient nation, a nation whose language thou knowest not, neither understandest what they say." — Jeremiah 5:15 (ASV)

Israel is not used here for the ten tribes, but for the whole house of Jacob, of which Judah was now the representative.

Mighty – “permanent, enduring.” The word is the usual epithet (of the rocks (Numbers 24:21) and of ever-flowing streams (Deuteronomy 21:4), Hebrew). It describes therefore a nation whose empire is firm as a rock and ever rolling onward like a mighty river. The epithet “ancient” refers simply to time.

Whose language thou knowest not – This would render them more pitiless, as they would not understand their cries for mercy.