Is the United States somewhat unique in its constitution that a defendant need not testify against himself or herself in criminal court (can remain absolutely silent from beginning to end), or is that fairly common among other Westernized nations as well?
IANAL, and not sure if it is in local constitution here, but it is probably written somewhere and you can sure as heck use silence to defend yourself (or take no defense or defend yourself or ask God to defend you). Judges and senates and jury takes notice and that is it. Then they judge according to other evidence.
And I wonder how they would you make testify against anything or anyone without some kind of torture (applies for democracies and similars only).
Some have it by Constitution, others by law or court rules; the EU has it as a treaty directive, many Commonwealth nations have it as a matter of precedent. So it is not US-unique.
This document identifies 100+ countries that have the equivalent of Miranda Warnings – not all include nonincrimination, and the document is more about what establishes the warnings themselves, but in quite a few it makes reference to constitutional provisions on nonincrimination.
Right to silence is protected by s. 7 of the Canadian Chartet or Righys and Freedoms. That right applies from first contact with the police to the end of the trial.
Right to counsel is protected by s. 10(b) of the Charter. Upon arrest or detention, the officer must advise of the right to counsel. (Note that this is a broader notice requirement for Miranda warnings.). If the individual asserts their right to counsel, the police must hold off on any interrogations until after the individual has consulted counsel.
It’s a fundamental principle of the common law; the drafters of the US constitution did not event it. It has since made its way into the constitutions of many other common law and non-common law countries, but even those which do not make it explicit in their constition will often still recognise it as an important principle.
The signficance of expressing it in a written constitution, of course, is that it tends to make it more difficult to limit it or override it by statute, and it exposes attempts to do so the the scrutiny of the courts.
Interesting, then, that the United Kingdom (Scotland excepted) does not respect it:
Canada only has it equivocally:
“You can be silent but the police can continue to hector and harass” isn’t exactly how it goes in countries where the right to silence is fully respected. Anyway, the link I just posted (for Canada) goes to the Wikipedia page on right to silence, which lays it all out for multiple countries all around the world.
I think the defining characteristic of the US 5th Amendment is that silence cannot be used against you. Some countries I believe have the right to remain silent and that silence can be used against you in Court.
The English/Welsh qualification to the principle - that if you raise in court something that you failed to mention when questioned, your failure to mention it can be pointed out in court, and the jury may draw inferences from it - is relatively recent, and was enacted by statute. You’re quite right that enshrining the principle in a constitutional bill of rights entrenches it and protects it from statutory derogations of this kind. My point is just that it wasn’t a novel principle in 1791; the US innovation was not the principle itself, but rather enshrining it in a constitutional bill of rights. But the great majority of common law countries now have a constitutional bill of rights - the UK and Australia being the most notable exceptions - and, since the privilege against self-incrimination is a widely-recognised common law principle, it turns up in most of them.
So the answer to the question raised by the OP - is the US somewhat unique in having this prinicple constitutionally protected? - is no, not unique at all. The US was the first country to do this, but is far from the only one to have done it. It might be closer to the truth to say that the UK and Australia are somewhate unique in not having a constitutional bill of rights in which the principle could be entrenched.
Japan technically had the same right BUT you don’t have a right to counsel during questioning and the police can hold and question you for up to 21 days.
After the 21 days, they simply arrest you on another charge and then they can hold you for another 21 days. Again, you don’t have access to a lawyer during questioning.
Rinse and repeat.
A lot of people give in to the pressure and sign false confessions simply to end the process. There’s an appalling number of confections where the only evidence is the confession.
China, IIRC, does this. Your silence or refusal to answer is indirect evidence of guilt.
Also you do still have the right to silence. The change is more honest, IMO. Juries are always going to consider something less truthful if it’s only revealed later.
But under the full-blooded version of the rule, if you had stayed silent when interviewed by the police, this is not something that would have been drawn to the jury’s attention. The jury might not even know that you had been interviewed by the police at any point. Whereas, now, the prosecution can suggest to the jury that your failure to mention this at interview is extremely significant.
It’s present in most Western legal systems. In legal literature (both domestic and comparative), it is often discussed or summarised under the Latin maxim nemo tenetur se ipsum accusare - nobody is bound to incriminate himself (the “accusare” is not limited to the formal bringing of an accusation, but includes any act that increases the chances of being prosecuted or convicted).
Nitpick: It is a bit repulsive 
to someone with an interest in EU law to hear of a “treaty directive” - “treaty” and “directive” are both used as terms of art in EU legal terminology, and they mean two very different things. But it is true that the maxim is part of EU law, both in the Charter of Fundamental Rights (which binds EU institutions themselves and Member States to the extent they apply EU law), nd the European Convention on Human Rights (which binds Member States comprehensively).
In Spain it’s kind of wider, you can simply refuse to answer without needing to invoke self-incrimination. No Miranda-style warnings, though; a frequent complaint of Spanish cops used to be that people who’d watched too many American cop shows had come to expect them. I undestand that a greater presence of Spanish cops in our televisions and movie theaters has helped with that, along with several popular cop series from Italy and Germany.
Conversely and simultaneously, there is a specific type of non-response which in Spain is treated as yielding, and which comes from Roman law: if you’re being sued. If you respond to any and all requirements from the court, attend the court sessions and refuse to answer, any other evidence must still be taken into account; if you don’t send back any paperwork received, any documents or other items requested and you’re a no-show, you lost. If you have only done part of it, there’s no clear directive in either direction but the judge is unlikely to be happy about you.
Norwegian trial law:
§ 123 then describes the right of any witness to refuse to answer questions that could incriminate or “profoundly embarrass” the witness or close family (as defined in § 122). The court can nevertheless require answers if it is considered reasonable after evaluating the nature of the trial, the importance of the testimony and the consequences to the witness.
I do not know how this actually works in practice.
In America there is a right against self incrimination, meaning you can and will be forced to incriminate others, for example journalists being forced to reveal their sources. The way it’s done is to get a court to order them to reveal the information and then convict them of contempt of court, which gives the court the right to imprison them for add long as it takes to coerce then into speaking. America does not have a right to remain silent.
I recall reading about the Napoleonic code vs. British common law. The USA and Commonwealth countries follow British common law, where a defendant cannot be compelled to testify. In Napoleonic law (spread over the European continent by some means in the early 1800’s, and IIRC based on Roman law) the defendant can be put on the stand and questioned and must answer; however, he cannot be charged with perjury for what he testifies in his defence. The principle is simple - a person can and will say anything to escape a conviction. Whereas, the British system goes the opposite way, saying you simply cannot force a person to testify against himself, but should he choose to then he does so at the risk of being prosecuted for perjury should he lie. An grant of immunity removes the need and right to avoid testifying, since it means you cannot be prosecuted for what you reveal - but the perjury laws still apply.
Of course, while under arrest but not in trial, you can say and do anything or nothing and it may or may not harm your case. But in the end, it all boils down to what can be admitted at trial.
I also recall some discussion about defendants jointly charged. Introducing the interrogation or words of one defendant would put the other at an unfair advantage, since he cannot force his partner in crime to take the stand and be cross-examined - thus the statement may not be admitted?
The recent British exception was to avoid the situation where months after the arrest the defendant pops up with an alibi. “If you were watching the telly with your gran, why didn’t you say so the day you were arrested?”
But many states have a journalist shield law, meaning a journalist cannot be compelled to reveal sources, because that could impact freedom of the press which is guaranteed by the first amendment.
By making it clear that it’s in your best interests. The trial of the Catexit Catalan-Parliament members has been quite interesting that way; the indicted expected that the head of the Mossos (Catalan cops), who is indicted in a different trial, would clam up. Instead he declared in the extremely clear and precise language that courts are used to expecting from cops. He listed names, he listed places, he gave precise times. He made it clear, without ever saying so, that his actions at the time were his best shot at “following direct orders from direct superiors while hoping as hard as possible that he wouldn’t end up going to jail for it” (the part before ‘while’ being what he’s indicted for).