I have noticed some people from Andalusia pronouncing the combination "st" in reverse in a word, as "ch" (for example "donde está" will sound like "donde echá"). Is it a particularity of the local accent of some kind of slang? In what areas, if any, can this pronunciation be heard?
This affrication of
/st/ is indeed particular to Western Andalusian:
An affricated dentoalveolar stop [ts] (listo [ˈlitso]) has been described as a variant of /st/-clusters. This sound is perceptually and acoustically similar to [th], another /st/-allophone in Seville Spanish.
The study reports that it more often occurs in younger, educated speakers, and in informal speech:
The frequency of occurrence of [ts] was negatively correlated with age and speech formality level, and positively correlated with the level of education. The social distribution of [ts] suggests a sound change in progress in Seville Spanish.
The study posits the novel sound change: [ht] ⟶ [th] ⟶ [ts] (noting that this affricate has not been described in the traditional dialectological studies).
Also note the distinction between Eastern and Western realization of /s/ aspiration when it occurs before consonants (pre- and post- respectively):
In Eastern Andalusian Spanish, /sp, st, sk/ clusters are usually realized with preaspiration [hp, ht, hk], whereas in Western Andalusian Spanish, postaspiration [ph, th, kh] is much more frequent, at least among younger speakers...