Due to word limitation: [ Part 1 ]
Nobody knows how to speak that language.
কেউ না জানে না ওই ভাষা কেমন করে কথা বলা হয় ।
=> কেউ জানে না (যে) ওই ভাষা কিভাবে / কেমন করে বলা হয় ।
They pretty much mean the same thing.
এদের মধ্যে এক অর্থ ।
=> এগুলোর অর্থ প্রায় একই ।
=> এগুলো প্রায় একই অর্থকে বোঝায় ।
Despite the fact that product is ubiquitous, no one can name it.
গুণফল সর্বত্র বিদ্যমান হইয়া সত্তেও, যে কোনো তার নাম বলতে পারে না
=> Sorry, I couldn't comprehend it well. Esp, the part - "~that product is ubiquitous~"
Where do you think you are going?
মনে করে তুমি কোথায় যাচ্ছ?
=> তুমি কোথায় যাচ্ছ বলে মনে কর ?
***There are sometimes situations in English when you say this phrase to someone whom you are speaking to in an insulting way in order to tell him that he/she shouldn't go anywhere 'now' (at the moment of the event) but rather has to stay where he/she is. It's said in somewhat this manner, "Aaaand...where do you think YOU are going???!" (<-with a the long 'a' in And). While speaking like that in Bengali, we often would say, "আর, তুমি কোথায় যাচ্ছ ?" . Try to lengthen the আ sound of আর and also emphasize on তুমি .