Indeed the most literally translation of verse 12:36 can be considered as:
And there entered the prison with him two young men. One of them said, "Indeed, I have seen myself [in a dream] pressing wine." ...
This is the closest to a word by word translation of the Arabic text, as "خمر" indeed is synonym of "wine". Even if the verse actually means to say:
And there entered the prison with him two young men. One of them said, "Indeed, I have seen myself [in a dream] pressing grapes." ...
Imam al-Baghawi in his tafsir explain this as follows:
(إني أراني أعصر خمرا ) أي : عنبا ، سمى العنب خمرا باسم ما يئول إليه ، كما يقال : فلان يطبخ الآجر أي : يطبخ اللبن للآجر . وقيل : الخمر العنب بلغة عمان.
My own translation take it with the necessary care:
("Indeed, I have seen myself pressing wine.") meaning: grapes, the grapes where here named by the name of that which they will end as, as is said: he is cocking (backing) bricks which means he is cocking (backing) adobe (to produce) for bricks. It was also said that wine is grapes in the language of 'Oman.
According to imam al-Qurtobi in his tafsir the interpretation of grapes being called wine in the language of the people of 'Oman is based on a statement of ad-Dahhak (this is also clearly quoted in tafsir ibn Kathir), al-Qurtobi added some more explanation saying:
. وقرأ ابن مسعود : " إني أراني أعصر عنبا " . وقال الأصمعي : أخبرني المعتمر بن سليمان أنه لقي أعرابيا ومعه عنب فقال له : ما معك ؟ قال : خمر ...
Again my own translation take it carefully:
Ibn Mas'ud has read it: ("Indeed, I have seen myself pressing grapes."). (The linguist) al-Asma'i said: al-Mo'tamir ibn Sulayman told me that he met a beduin who had some grapes and asked him: What do you have with you? He answered: wine.
So basically we have two options: The verse means what ever has wine as a result, or an indirect connection to grapes due to a special variance of colloquial or regional use in the Arabic language itself. However in both cases it is more likely that wine refers to the grapes.