The language used in Cymatic software can be chosen from the list in the settings (File Menu / Settings / Language tab).
To force Cymatic to use a particular language, choose it from the list, then click "Save as Default", then close the program and re-open it.
If you don't choose a language in Cymatic, then the software automatically uses the same language as chosen in Microsoft Windows. If Cymatic has not yet been translated into your Windows language, then English will be used.
Your Betfair account also has its own language which determines the names of events and markets.