I can't understand, speak, or write much bengali and can speak understand and speak Sylheti quite well -though it's not much of a current written language any more.
I think it's best to focus on Arabic and English since these to are the most useful in terms of ad-Dunya and al-Aakhirah and are familiar in all parts of the globe.
So many intellectuals now use Arabic and English in material and Spiritual treatises that it's wise to jump on the bandwagon ahead of the game since most other languages are going obsolete even in their motherlands.
English is all over soap bars and light bulbs and toothpastes and most other commercial equipment and interestingly has the most number of Quran interpretations on the planet.
Arabic is a must for anyone who seeks to understand what they are praying and how to conduct themselves as the best of creation - above animals and heedless humans - and to be able to absorb the depth and intricate detailed dimensional texture of the Quran - unless they want to be categorized with parables to donkeys and drunkards by Allah
So in sum - the only use i see in languages other than Arabic and English are their transient communication value, and the need for their preservation for the sake of reference - a bit like how heiroglyphics professors are needed in order to understand certain details of ancient Egypt.
I project that Arabic and English are the languages of future generations and investment in these will streamline progress and make brain memory usage efficient - although Arabic needs the "pa" after the "ba".
My fav bollywood accent:
I'reely think you nouvat i'am saying - haqq eemaandaari aur muhabbat se.