When we tell about a culture, we'll usually note traditions and customs. --> When we talk about a culture, we usually note traditions and customs. When we say "tell about", we usually mention the object. For example, "He told me about his promotion". "He" is the subject, while "me" is the object. Because there's no object here, we can use "talk" instead. "We'll" means "we will", and usually refers to the future tense. "I will go out" or "He will regret it". Because the first part ("when we talk about") is in the present tense and you're talking about a general event, we need to use the present tense in the latter part of the sentence, so it will be "we usually...". The rest of that sentence is perfect! It's important to know, what influences on the culture. --> It's important to know what influences the culture. This sentence is nearly perfect; firstly, that comma shouldn't be there. There are certain rules for commas and I can't go through them all, but in this case, you don't need a comma. [If you look at my previous sentence, I used a comma twice. One before "but", because commas usually go before connectives to separate two independent clauses, and another after "in this case", because commas go after adverbial phrases (e.g. therefore, tomorrow, however, when I came back)] Secondly, you don't need to use "on" after "influence", because it means to have an effect <<on>>, so that is already implied. You have two options here: you can say "what influences the culture" or "what influences there are on the culture". The latter turns "influences" into a noun, while the former uses it as a verb. Is there the cultural diversity or multiculturalism in a country? --> Is there cultural diversity or multiculturalism in a country? We don't use the article "the" here, because we aren't talking about something specific. For example, you would say "Did you put the sugar in the tea" because you're referring to a specific cup of tea, perhaps one already made.

I think it's because they're in the plural. Who is Obama? He was the US President. Who are Bush and Clinton? They were US Presidents. That last example is grammatically correct. You would say "I want to buy a shirt" not "I want to buy shirt".

"Suggest" means to put an idea for consideration, or to "throw it out there". For example: Your car is a bit old. I suggest you buy a new one. I am mentioning an idea or recommending something. It is similar to "I would [say]", which is used when you're giving a personal opinion.