Financial-Modelling.net, as many other websites about Excel, uses the English names of the Excel cell formulas. If you use Excel in another language, you will not be able to simply copy and paste the examples from the text; perhaps some of the examples are even much harder to understand if you are unfamiliar with the English function names. However, the University of Twente offers a useful overview over the international names: http://wwwhome.ewi.utwente.nl/~trieschn/excel/excel.html
With increasing complexity, financial models become more prone to errors. While you can never be sure if a model is completely correct (let alone accurate), you can avoid mistakes with a few easy best practices.