Learning Kathak is first and foremost learning about a new culture. This is not just modern North India of Delhi and Mumbai, but the "traditional" culture of India. You won't be able to recite poems/kavittas if you don't train your tongue to learn Hindi pronunciation, you can't memorise the names of the arm movements/hastaks if you don't become familiar with Sanskrit.
Kathak is not just poetry and stories, it is also music! Rythmic patterns, melodies, pitches, instruments are radically different from Western music...and that's the beauty of it!