Teaching mathematics in children's first language has both cognitive benefits and assists with developing cultural and mathematical identity. In Papua New Guinea, many different Indigenous languages are used for instruction in elementary schools and teachers often need to identify or develop mathematics terminology themselves. Building on prior research, guidelines were developed to assist teachers to find the mathematics in their own vernacular languages. The guidelines were adaptable to different language ecologies and covered mathematical areas such as counting systems, measuring and comparing, location and shapes. The guidelines were applied by teachers in professional development workshops. The paper shows examples from some of the workshops of the challenges that the teachers faced in trying to identify appropriate mathematical language, and that contributed to the refining of the guidelines.