Learn English – Are the words “whine” and “wine” pronounced the same

Short answer: "zh" and "j" are not pronounced in the same way.

Using the International Phonetic Alphabet, the "zh" digraph would be transcribed as [ʒ], while the "j" letter would be [dʒ]. The "zh" sound occurs at the beginning of the name "Jacques" [ʒɔk], and in the middle of the word "leisure" [liʒɚ]. The "j" sound is two IPA symbols because it is a combination of two sounds, [d] (as in "dog") and [ʒ]; [dʒ] occurs at the beginning of the word "jock" [dʒɔk].

The "zh" and "j" sounds are definitely not identical to an English speaker. In fact, two words could be distinguished only by the difference between those two sounds. The following would represent such a minimal pair:

• "legion" [lidʒən] (where the "gi" sound is identical to "j")
• "lesion" [liʒən]

("Jock" and "Jacques", which I already mentioned, also only differ in that one sound.)

The reason you see many borrowed words using "zh" is because English lacks a standard letter to represent the [ʒ] sound. In French borrowings, it is "j", because "j" is always [ʒ] in French. Sometimes it is "si" (as in "lesion") because of a natural phonological process that occurs in English. So, there is no letter that always gets pronounced [ʒ]. But, in many languages that don't use Latin script, there is a distinct letter for [ʒ]. In these languages, "zh" is often the standard way to translate the sound [ʒ] from their orthographical system to the Latin one in an unambiguous way.

As to whether you can use "zh" to imply "j" — I am not certain what you mean by "imply", but since these are two different sounds, I think the answer is probably no.

The allophones of /t/ in English are [tʰ], [t], [ɾ], and [ʔ]. Which of those you get in any particular word and speaker depends on many, many factors.

Both trader and traitor alike are indeed pronounced [ˈtʰɹeɪɾɚ] by most North Americans, particularly in casual or quick speech.

Intervocalic /t/ almost always reduces to a single flap [ɾ] there. That’s why ladder and latter are homophonic, although kitten and kiddin’ are not. Indeed, kitten may become just [ˈkʰɪʔn̩] (sometimes written [ˈkʰɪʔən]) , often enough.

In contrast, intervocalic /t/ does not reduce to a flap in RP; it can sometimes do so in other British dialects, though. So RP would make traitor into [ˈtʰɹeɪtə] instead, with a linking [ɹ] at the end as needed for liaison only.

In Scots English you might find [ˈtɾeɪtʰɚ] (sometimes written [ˈtɾeɪtʰəɹ]), though, with now the initial ‹r› converted into a flap instead of the ‹t›. Just depends on the speaker.

See here for innumerably many other fascinating details and distinctions. In particular, see for example better and daughter. (Just don’t take too seriously the uptalking teenaged boy they got for the General American; that sort of high-tone rising is not commonly heard in older speakers. It has a very “valley girl” sound to it.)