Almost no one in my area of the US is able to pronounce the "th" sound, let alone the "ths" sound, so people learning English really shouldn't feel bad about having trouble with it. Most of the people around here are children and grandchildren of Polish and German immigrants from between WWI and WWII, so there is still a lot of Polish spoken (the Germans integrated a lot faster) and there is a strong influence on the English that we speak here. Younger people, who grew up with more TV and radio and such, tend to have less of an accent and more luck with some of the more difficult English sounds (I can say "the" or "sixths" correctly, if I think about it before I say it) but generally you will hear people pronouncing the "th" sound like it is halfway between "t" and "d." I've pretty much given up on ever losing the accent.