why a sudden change of language?

Indeed, the change of language is logical and necessary to make the film believable.
You have to remember that Fanis is born as a member of the greek minority in Istanbul in Turkey and his mother tongue is greek. Saime on the other hand has turkish parents and speaks turkish as native language.
I don't speak neither greek nor turkish, so I can't say in which language they spoke to each other when being children, but should think that turkish more probable, as a minority usually even speaks the majority's language. But it is even possible that they spoke greek, as children can learn other languages very fast and maybe Saime learned greek when being at Fanis' place. But that is of no importance, the important thing is:
When being about 7 years old, Fanis and his family (and most of the other greeks in Istanbul) were deported to Greece, where all people speak greek and he had no opportunity at all to speak or even hear turkish (the enemys language). SO HE FORGOT TURKISH. When not using a language you forget it very fast, and even faster when you are just a little kid. (And even if he hadn't forgotten turkish, it would have been the turkish of a seven year old kid.)Which means that of course he had to speak english when returning to Istanbul 35 years later.
Saime -if she once could speak greek- forgot that language likewise, having no one to speak that language whith. So they HAD to use english when Fanis returned, even if they once had been childhood sweethearts.

English is like Greece's second language. In Greece, it's necessary to know 2-3 different langauges. My friend Irini, who lives in Athens, speaks Greek, English, French, and Italian!

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I think it is very logical and what would logically happen.

Remember that Constantinople used to be Greek and early 20th century had a big Greek population under Ottoman rule. So the Greek element was well preserved and the Greeks of course spoke Greek as the Turkish, Turkish.

Fanis' parents which had lived in Constantinople all their lives were bilingual and could speak Turkish as well.

Fanis however had left Constantinople when he was young so never learned to speak Turkish. Thus when he returned later to Constantinople the language spoken between him and the Turks is English as they don't know each others language.

Which is the strange thing? Fanis was about 35-40 years away from Turkey, he never spoke Turkish from then and it is logical to forget the language. The three different languages are used to make the movie more real.

Jenna you must be Greeks leaving abroad to speak English in your company. I can't imagine Greeks here talking to each other in English, this just sounds redicoulous.

I just got back home from the movie theatre, where I saw this wonderful movie. I must say the sudden change of language appeared a little bit odd, to me too. But now I'm thinking about the characters and I do remember them saying a few words or even a few sentences in another language (other than English) in the part of the movie they where mostly speaking in English. I remember him saying that he never went back because he was afraid and I think that they can't speak in greek or turkish because that would be going back to their childhood and, of course, they are frightened. But I do remember them not talking in English every now and then, but they continue talkig English, for example, when Saimes (ex)Husband comes back... I don't know... it was odd, at the beginning, but it then made sense, somehow... because talking in English is like looking trough that mirror, on which you see where you came from and that picture that remains as a promise... a promise that you know that will not bloom.

Well, let me give you some insight, being Greek myself.
First of all, I believe that this film is one that should have been recommended for an Oscar, not that I care about the Oscars terribly, but I mention it just for the record.
The film takes place in Constantinople - or Istanbul as it is widely known - before the Turkish invasion in Cyprus, that is the late 60s beginning of the 70s. Greeks living in Turkey had to speak the turkish language for obvious reasons, but their mother tongue was greek. In the film, when Fanis is still a little boy, his grandfather and friends, and the grown-ups in general, when they need to have a serious - usually political - discussion in front of the child, change from greek to turkish, hence pointing that little Fanis could speak turkish, but not to a very sophisticated level. Also, when Fanis' parents argued, they started in greek and changed to turkish.
Regarding the rest of the film when Fanis grows up to become Giorgos Coarraface (my personal favourite) and meets up with Saime and speak in english, I believe that they do that for a) practical reasons and b) it is proved throughout the film that little Fanis doesn't speak very good turkish anyway, so as a grown man has probably forgotten all he knew. It is quite true that the older generation of Greeks from Constantinople speak turkish very fluently, but the younger generations don't.

I hope this shed some light!

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IMO it's kinda natural thet they both speak in english because if you don't speak a language for a long long while, you might forget it almost totally. I meant it's not like riding a bicycle!

Fanis, as some greek posters have mentioned, speaks turkish only during his childhood and just with turks (like Saime). Later in Greece he doesn't speak turkish anymore.

I have relatives who moved from México to Québec, Canada. When I visit them or whenever they comes to my town during holidays, I can only speak in spanish with the elder ones. With those who left México being kids I can barely speak in spanish, So we use mostly english and a bit of spanish and french.
The ones who were born there can't speak spanish at all, they can understand a bit and that's all, hence we gotta talk to them either in english or french.

IMO wouldn't be credible if Fanis could speak perfect turkish after 40 years without speaking it.