If the handwriting recognition shows a wrong default language, please navigate to Options> Settings> General> Default Language and check whether your language settings are correct.
If they are correctly set, and the language you see in the pop-up after converting your handwriting to text is still wrong, you will need to change the language for the particular notebook. Tap on the three dots (...) in the upper right of the toolbar and open "Search". At the bottom, you will see the language setting. Change this to the desired language, and the whole notebook should display the correct language when searching or converting handwriting to text.