What a fascinating question, Flam! And a great discussion everyone.
From my regionally-biased observations speakers of Romance, Arabic and Indic languages tend to use T whereas Japanese, Chinese and Korean speakers resort to S.
Let's expand and condense this slightly:
The tough consonants for most foreign speakers are the inter-dentals, both unvoiced and voiced:
[th] (thorn) and [dh] (edh)
These are often replaced by [t] and [d] or [s] and [z] But sometimes these two are replaced with [f] and [v] (notice that the voicing in consistent with English sound being substituted).
That thing thinks that those thighs thicken.
Dhat thing thinks dhat dhose thighs thicken.
with French (or German?) accent:
Zat sing sinks zat zose sighs sicken.
Dat ting tinks dat dose tighs ticken.
It is hard for me to reproduce any more accents. I'll have to listen for some more, but the underlying question still remains:
Why are (foreign) accents so consistent within a linguistic group? Why do the vast majority of the speakers of a certain language prefer one substitution over the other even when both pairs of phonemes are available?
Why do the French almost always choose [s] and [z] to replace [th] and [dh], when they certainly have [t] and [d] in their repetoire of sounds?
Just as a Norwegian chooses [t] and [d], when they would have a choice of their native [s] and [z]?
There are dialects of English that do avoid [th] and [dh] by using [t] and [d], but there are none that substitute [s] and [z].
'tree 'yout' 'dey' 'dat' 'broder' 'moder' (I didn't change the rest of the phonology to match, but you get the idea!)
A curious thing that I noticed about one of my mechanics (native speaker of English with an English mother) is that he would use [th] and [dh] in the initial position but would substitute [f] and [v] in any other position.
The other day I went with my brother to the brothel.
Dhe over day I went wif my brover to the brofel.
I am wondering if the substitution goes anything like that in Portuguese, BD!
Spanish is sort of a mixed bag, because the [d] of Spanish is very dental and especially fricative between vowels, so the main problem is initial. Also it is in most of the Spanish-speaking world's "accent repetoire" to make the [th] sound when making fun of Spaniards.
Arabs shouldn't have a problem since they have both the letter 'tha' [th] and the letter 'dhal' [dh]. Although having the sound doesn't always mean they are used correctly in English. An Irani friend of mine used to do something with his 'w' and his 'v', though he said them both correctly he switched them around for some reason.
Why do the speakers of one language "hear" [s] and [z] as replacements for [th] and [dh], while speakers of another language "hear" [t] and [d] (or even [f] and[v]) as the better substitution, even when both languages contain either option?
Darn good question, Flam!
'Experiments are the only means of knowledge at our disposal. The rest is poetry, imagination.' -Max Planck