In most english translations of the Qur'an, the second ayat of surah Baqarah begins with the words 'this is the book' instead of 'that is the book'. The arabic word translates to 'that' rather than 'this'. So why has a different word been used in this case?
It's not a matter of overlooking. It was translated that way mainly because that is a common interpretation of what the Arabic word "Zaalika" means in that context even though usually it means "that" rather than "this".
In the tafsir of that verse, Ibn Kathir says, for example:
قَالَ ابْنُ جُرَيج: قَالَ ابْنُ عَبَّاسٍ: "ذَلِكَ الْكِتَابُ": هَذَا الْكِتَابُ. وَكَذَا قَالَ مُجَاهِدٌ، وَعِكْرِمَةُ، وَسَعِيدُ بْنُ جُبَيْرٍ، وَالسُّدِّيُّ وَمُقَاتِلُ بْنُ حَيَّانَ، وَزَيْدُ بْنُ أَسْلَمَ، وَابْنُ جُرَيْجٍ: أَنَّ ذَلِكَ بِمَعْنَى هَذَا، وَالْعَرَبُ تُقَارِضُ بَيْنَ هَذَيْنِ الِاسْمَيْنِ مِنْ أَسْمَاءِ الْإِشَارَةِ فَيَسْتَعْمِلُونَ كُلًّا مِنْهُمَا مَكَانَ الْآخَرِ، وَهَذَا مَعْرُوفٌ فِي كَلَامِهِمْ.
Ibn Juraij reported that Ibn Abbas said: "ذَلِكَ الْكِتَابُ" means "هَذَا الْكِتَابُ" ("this book").
Likewise said Mujahid, Ikrimah, Sa'eed ibn Jubair, As-Suddi, Muqatil ibn Hayyan, Zaid ibn Aslam, and Ibn Juraij: that "ذَلِكَ" is being used with the meaning "هَذَا" ("this"). And the Arabs interchanged between these two pointing pronouns using either of the two in the place of the other, and this is well-known in their speech.
Translation is a type of interpretation, and many translators decided to use this interpretation of the verse.
In addition, the purpose of a translation is to make it easier for people to understand. Saying "that book" in English might confuse people even though it might not confuse in Arabic.
The message to get across is that Allah is talking about the Quran in this phrase. People reading "that book" in English might get the wrong idea.