A source: https://deykun.github.io/diffle-lang/cs?p=about-language (It has tooltips displaying percentages for other letters)

I thought that a was the most popular letter.
Also the n is probably in part due to the prefixes na-, ne-, and nej-.

Zajímavé, tomu se mi skoro nechce věrit, že by tolik českých slov začínalo na N. (Interestingly, it’s surprising that there are so many words beginning with N in Czech. )

Ne, ne, ne, ne, ne!