In this particular case, the subtle difference between "is" and "are" can easily be rendered in Russian: "the audience are watching" - "зрители смотрят", "the audience is watching" - "публика смотрит". By the way, we more often use the word "спектакль" than "пьеса" as the object of "смотреть". Обычно мы читаем пьесу, но смотрим спектакль.
Hmmm. Technically, "audience" is singular, but I too answered: "are watching". It's always a bit tricky with nouns designating a lot of people (the company, the crowd, the group, the class etc.) Should it be: "The government is" or: "The government are"? Collectives like governments and audiences aren't disembodied entities, so sometimes it feels correct to say: "are". I don't think I have a firm rule on this, except to stay consistent within the same piece of writing. If I start off with the plural form, I have to make sure not to revert to singular halfway through.
For the benefit of English non-natives, and probably many a native as well, this is an excellent description of verb conjugation for subjects like 'audience'.
Grammatically, whether one chooses "the audience is" or "the audience are" indicates something about the unanimity with which the speaker conceives of the audience as acting. In practice, in this case, it probably simply says more about the geographic origins of the speaker.
However, I'd like to add that in English, 'audience,' while it is a collective noun, can also be made into a plural: 'audiences.' With the first, you can use either "is watching" or "are watching;" with the second, of course, one needs to always use the plural verb forms. Personally, as a native speaker of American English (and a trained linguist), I would say "the audience is watching." But I can see how this could vary.
It may be worth noting, however, that one can switch freely from one to the other, e.g.: "The audience is watching the play; the audience are beginning to twitch in their seats at the three hour mark." In a way, as you say, Adam, it's a matter of whether you perceive the audience to be one single entity, or a number of individual people.
Turkish has it too. "Piyes". It's nowadays more used for short amateur productions, school plays, etc.
But we don't use the word 'spectators' for those watching a play or a musical performance - not in British English, anyway.
I think of 'spectators' as those watching a purely visual performance, such as a sporting event.
Plays and musical performances not just watched, but listened to, and we use the word 'audience' in relation to those watching them. (With television, that becomes 'viewers' for those watching at home, but 'audience' for those who are present at the live studio recording.)
The primary definition of audience is the set of all spectators, so this winds up being a bit of a weird sentence. I am not confident in my understanding of the nuances of the word "зритель"; it seems like it primary means "a single audience member" as opposed to a whole audience, e.g. here.
Yeah, that sounds a bit weird. Technically, mulitple audiences - which is what your sentence implies - cannot watch the same play at the same time. They can watch different productions of the same play, but not the same play, since by their very nature of being plural audiences, they cannot be in the same auditorium or hall at the same time. You would have to say, "The audiences are watching plays" for this to truly work.
On the other hand, you CAN say, "[The] audiences love the play," meaning that multiple audiences have seen a given play at different times, and they all love it. So, that sentence structure actually works grammatically in and of itself; just not with the particular verb "to watch."
The rules on whether to use singular or plural with collective nouns vary from language to language. In my native portuguese, with a (singular) collective noun the verb should be in the singular. There might be exceptions, and it is a common mistake even in native speakers, but it is usually considered a sign of poor education to use singular collective with plural verb. Of course with plural collectives (e.g. audienceS) the verb should be plural.