Why do non-native English users often spell "standard" as "standart"?

This is a common misspelling in German. The word also is "Standard" there, but as @Tim points out, "Standard" and "Standart" sound the same, and it is easy to mistakenly assume it's related to German "Art" (Way, manner, fashion).

I think this is a case where an error in the native language is repeated when those making it write in English.

I don't think it's a very common mistake in general. In the case you quote, it is probably because standard is Стандарт in Russian, which ends with a т.

In some languages, such as Russian and German, voiced consonant sounds at the end of a word change into their unvoiced counterparts, such as d to t and g to k. There is no Russian word ending with д and a d sound.

In Czech language, "standarta" means flag. "Standard" is commonly misspelled as "standart" even by native speakers, because they don't see the difference.