There are only six integers between 1 and 1,000,000 whose English names contain six letters: ELEVEN, TWELVE, TWENTY, THIRTY, EIGHTY, and NINETY.

As it happens, the same is true in Spanish: CUATRO (4), QUINCE (15), VEINTE (20), MIL UNO (1,001), MIL DOS (1,002), and DOS MIL (2,000).

(Thanks, Claudio.)