Last one - 18d. Local language error keeps on returning (7). I have e-p-n-l and I think they're all right! All I can come up with is empanel which doesn't make any sense.
23a. Means to catch some sleep, which is ok in certain circumstances (8) -g-e-e-- And 19d. American students thick? That's about right (4) -r-- Last two but I can't work them out!