כִּ֚י בְּלַעֲגֵ֣י שָׂפָ֔ה וּבְלָשׁ֖וֹן אַחֶ֑רֶת יְדַבֵּ֖ר אֶל־הָעָ֥ם הַזֶּֽה׃
For with stammering lips and with a strange tongue Shall it be spoken to this people;
Rashi on Isaiah
With distorted speech (בְּלַעֲגֵי שָׂפָה) Similarly, לְעָגִים and (infra 32:4), עִלְּגִים, are both expressions of distorted speech, which is not readily grasped.
Ibn Ezra on Isaiah
בלעני שפה With laughing12A. V., Stammering. lips. Comp. לענ derision (Ps. lxxix., 4). The teacher speaks to the child in a laughing and mocking way.
Rashi on Isaiah
he speaks to this people Everyone who speaks to them words of prophecy or admonition is to them like a distorted language, which they cannot understand.