Question 1:
Is the Igbo language spoken in any country other than Nigeria?
Hausa for example is the official language of the country of Niger, and it's also spoken in the northern parts of Nigeria. It is also spoken by by muslim populations in other West African countries such as Benin, Togo, Ghana.
Same is true of Yoruba language spoken in Bening, Togo, and as far away as Brazil and Cuba.
Question 2:
Since Nigerian languages have many dialects, do speakers of these dialects understand each other?
For example, does a Lagos Yorubaman understand an Ijebu Yorubaman? Does an Onitsha Igboman understand the Owerri Igboman? Does the Sokoto Hausaman understand the Makurdi hausaman?