Both Revised Standard Version and Good News Bible have a paragraph break here, which we recommend.

When the Jews heard of Nicanor’s coming and the gathering of the Gentiles: The gathering of the Gentiles refers to their rushing to join Nicanor’s forces in the previous verse. For this whole clause Contemporary English Version has “When our people found out that Nicanor was coming to attack with all of these Gentiles.” We may also say “When our people heard that Nicanor and all those Gentiles were coming to attack them.”

They sprinkled dust upon their heads: See the comments on 1Macc 11.71 and 2Macc 10.25. Here their heads is not specifically said in the text; Good News Bible says “themselves,” which is just as accurate a translation.

And prayed to him who established his own people for ever and always upholds his own heritage by manifesting himself: The pronoun him refers to God. The phrases his own people and his own heritage both refer to the Jewish people (compare Deut 32.9). Established his own people for ever means God chose the Jews as his people and gave them a permanent place among the nations of the world. Always upholds his own heritage by manifesting himself means God does what is necessary to defend his people. Good News Bible expresses this last half of the verse well with “and prayed to their God, who had chosen their nation as his possession forever and had never failed to help them in time of need.”

Quoted with permission from Bullard, Roger A. and Hatton, Howard A. A Handbook on 1-2 Maccabees. (UBS Helps for Translators). New York: UBS, 2011. For this and other handbooks for translators see here.