Student or Learner
Do we say:
Did you buy all the Japanese newspapers
Did you buy all Japanese newspapers
Note that a better title would have been Japanese newspapers.
For a start both questions should end with a question mark.
'Did you buy all the Japanese newspapers?' means 'Did you buy every newpaper printed in Japanese?'
'Did you buy all Japanese newspapers?' means 'Did you buy all newspapers printed in Japanese and in no other language?'