Languages cap are at 9. If you learn 10th language (and more after), you will be able to understand it, but you will not be able to speak it. 10th language will not appear in the drop down menu for the known languages because the window of language list is too small. The languages are listed in the order they were assigned to the player character. To fix this issue, add scroll bar or increase the size of drop down menu for language selection options.