Bengali should remain the only official language of Bangladesh. In a monolingual country like Bangladesh, there is no justification to give official status to any other language. Not even English. English should be taught from childhood as a subject, but no official recognition.
Now come to Arabic. Arabic is perhaps the most absurd language in the world. Currently there are 22 Arab countries, they speak wildly different dialect of Arabic which often not mutually intelligible from country to country. For example, Arabic spoken in North Africa(Morocco, Algeria) is totally different from Arabic spoken in Arabian peninsula or Levant. More bizarre thing about Arabic is that Arabic uses in written from do not match with Arabic spoken by common people. The Arabs use classical Arabic for writing purpose which was used by Quraish tribe 1400 years ago, the long extinct dialect in which Quran was written. Current spoken Arabic is totally different.
And the bizarreness do not stop here, the Arabic is written from right to left, yet the numerals are written from left to right. It is particularly problematic when someone study mathematics in Arabic language. Moreover Arabic language which is written without tazwid is very difficult for any non Arabic speaker to correctly read. This Arabic script problem was one of the reason many Muslim countries(Turkey, Malaysia some others) ditched Arabic script in favor of Roman script in the beginning of 20th century.
The spoken Arabic vs Written Arabic problem is also one of the major factor of low level of literacy and intellectual bankruptcy of the Arab world. Some Arab countries like Morocco, Algeria, Tunisia are ditching Arabic in favor of French. Then why should any sane country will adopt this problematic language as an official language for no reason? just because it is considered Islamic language? But there is no rule in Islam which requires making Arabic as an official language to become pious or worthy of entering paradise. Learning Salat and some Duaa and be able to read the Quran in Arabic is enough for common Muslim. No need to burden our children requiring them to learn some worthless language which will be no use in this world and is not mandatory to shine in afterlife. Thanks.