Online testing is becoming a popular way to deliver language tests, partly because of its reduced cost, partly because of the high quality of test data collection. In language tests, interface validation has received a limited attention in professional literature (García, Magal, da Rocha, & Fernández, 2010). This paper will show the validation process of the OPENPAU application, which aims at language testing exam delivery through an attitudinal study. The real importance of this project is that it offers reliable solutions at a low cost for the needs of a nationally delivered online test that can serve to assess all the traditional language skills (namely speaking, listening, reading and writing) in an efficient, simple and cost-effective manner. Our paper presents the design, development, and evaluation of a tablet PC software application for assessing both productive and receptive skills in foreign languages for its prospective use in the Final Test of Baccalaureate. The software development has been carried out within the OPENPAU project (FFI2011-22442), funded by the Ministry of Economy and Competitiveness. [For the complete volume, "New Perspectives on Teaching and Working with Languages in the Digital Era," see ED565799.]