The meaning of the phrase "First language" is-

The meaning of the phrase "First language" is-গ) a person's native language.

First language:
English Meaning: a person's native language / the language that someone learns to speak first.
Bangla Meaning: সগোত্রের ভাষা / যে ভাষায় সবার আগে কথা বলতে শিখে।
Ex. Sentence: We speak Bangla as our first language.

First and foremost:
English Meaning: most importantly, more than anything else.
Bangla Meaning: সর্বাধিক গুরুত্বপূর্ণ; প্রথমে এবং সর্বাগ্রে।

Source: Merriam-Webster Dictionary, Accessible Dictionary, Cambridge Dictionary.