Oral descriptions of mathematic phenomena are indispensable for mathematic conversations in primary school but pose challenges for learners on a conceptual and linguistical level. Digital media offers potential to foster the oral descriptive skills, but the application of digital language scaffolding in mathematics has not been researched yet. Therefore, a digital scaffolding tool addressing primary school learners' oral descriptions of mathematical patterns and structures using the app 'Book Creator' is being developed in a design research project following the Integrative Learning Design Framework. This paper reports on teachers' evaluation of a prototype and its already implemented design principles, in order to identify implications and suggestions for the development in further micro-cycles. Results show that the current version is considered to be fruitful for primary school learners already but should be revised concerning the linguistic level and the cognitive activation.