Reading their various scriptures, reading books written about their scriptures, asking adherents of the faith, going to their worship houses and so on.
Knowing the languages certainly does help and I have no objection to this. It is, however, time consuming and some people may not simply have this time due to work, children, disabilities and so on. There is nothing wrong with, as you have done, asking people who do speak the language for an accurate meaning and rendition of a text.
I like that you read the Tanakh. You are under no obligation to uphold Torah Law, just so you know. This is, of course, according to my faith. You are obligated to follow the Noahide Code. I know that you mostly already do this, just by virtue of being a civilised human and a monotheist.
I do not care for these texts (Gospel and Qur'an), but I have no objections to their monotheism. I note you say 'Gospel'. I assume you do not like Paul?
I hope you do well.