Slavic speakers sidestep saying the “th” sound and slide over to the “z” sound. Using any word being with “th” can be a challenge and the results while not unpleasant can noticeable. “Will “zay” be “zair” or “day” be ”dair”.
Slavic languages for the most part operate on the front of the palette using the tongue, much like French. Yet the use of the tongue up against the top front teeth is still foreign to one who speaks Russian. The language has no identifiable article like “the” or “a”. Thus the necessity to include an article in speech plus the annunciation is difficult to coordinate.
I have noticed that in English we must exert a necessary step to accomplish the use of the “th” sound. It is necessary to place the tongue up and against the front teeth and bring the corners of our mouth up into an almost smile to accurately enunciate the sound.