Translation:German is a useful language because Germany is an important country.
'Tír' is feminine, as indicated by the 'í' in 'is ... í an ghearmáin'. So why is 'tábhachtach' not lenited?
You're correct it should be tír thábhachtach. While in some dialects tír can be masculine, it's more common for it to be feminine which it has been throughout the course. What's given here is probably a mistake.
Another small point, but the í in this sentence actually refers to An Ghearmáin. If the country in question were masculine, say Meiriceá then the sentence would read Is teanga úsáideach é an Béarla mar is tír thábhachtach é Meiriceá.