The phoneme 'th' does not exist in Spanish. So, we tend to mispronounce it. Many of us, at least for the native European Spanish speakers, pronounce a Spanish 'z' instead of 'th'.
By the way, the spanish phoneme 'z' is pronounced differently to the english phoneme 'z'.
Actually the sound /ð/ exists in Spanish, but it is an allophone of /d/. For example, the word dedo is pronounced as /'deðo/. That means that for a native Spanish speaker it is very difficult to learn to separate both sounds.
"The phonemes /b/, /d/, and /ɡ/ are realized as approximants (namely [β̞, ð̞, ɣ˕], hereafter represented without the downtacks) or fricatives[6] in all places except after a pause, after a nasal consonant, or—in the case of /d/—after a lateral consonant; in such contexts they are realized as voiced stops.[7] (In one region of Spain, the area around Madrid, word-final /d/ is sometimes pronounced [θ] especially in a colloquial pronunciation of its name, Madriz ([maˈðɾiθ]).[8]) "
And, if the case of participles, we just nearly butcher the in-between 'd'
in -ado as -ao, simillarly to the Southern speakers from the US on lots of words.
By the way, the spanish phoneme 'z' is pronounced differently to the english phoneme 'z'.