Happy new decade?

Being a native Spanish speaker, I am very clear that the decades (and centuries, and millennia) begin in years ending in 1, like 2021. This is because historians do not have a year 0 (zero). The Christian Era, therefore, begins in year one (and the year before that is year one before Christ, or -1).

Consequently, the first decade, the first century, the first millennium, they begin in that first year, number 1. In the long run, this means that all decades, centuries and millennia begin in a year ending in 1. But it seems that speakers of other languages, such as English, are not clear about this or simply do not consider it and — curiously enough, at least for me it is really curious — they introduce a first decade of nine years, a first century of 99 and a first millennium of 999, so that the second decade begins in the year 10, the second century in the year 100 and the second millennium in the year 1000…

I prefer a homogeneous accounting, and that all decades have the same number of years, in this case 10, and the same with centuries and millennia. A decade of nine years seems to me to be a contradiction. What do you think?