We used to have two equal "best answers", but that actually causes a loooot of angry/confused comments. We decided to treat the 'more American' version as the best and the number of some comments lowered significantly.
So the 'best translation' of "obiad" is "lunch", and I guess if you create your sentence from tiles then you may not have "dinner" as a choice. We don't control the 'distractor' tiles (the tiles with words not used in the best answer).
If you type your answer from a keyboard, then you can write "dinner" and it will be accepted.