using the word deeply

Deeply is fine.

the language--or what word could I use for 'deeply.'

But Swal do listen to Cen, he is positively brilliant, helps everyone, is right 99.99872% of the time

Tho he evidently hates some of us....

Quuote swallowtail:

I would rephrase that. When you say "the best way to know deeply the culture you are visiting" implies a quick process, since most visits are fairly short. It takes a long time to learn a language, so I would change the wording. Also, although deeply is a passable choice of word, it is most used in close personal relationships, like "The woman understood her husband's relief deeply". The less personal word used to convey understanding is "thoroughly".

Therefore, I would suggest: