In bug 35186 the issue was discussed that a language which has been defined per direct formatting is overridden when the paragraph style is changed. That's the expected behavior (see c6, c11, c17) but not really clear to the user since the paragraph style dialog has no means to change the language property. This is being propose in c13.
Setting to NEW. I support that as said in bug 35186.
No need for input from UX.