This isn't British, and it isn't slang. In general, most English-speakers don't worry about blasphemy these days. 'God damn' would be an odd thing to say in this context though. On its own, it isn't a common expletive. You could use 'god damn(ed) ' as an adjective, as in 'It's raining again and the goddamned tent pole has broken' There is 'God damn it' as an expletive, as in 'God damn it! It's raining again', or you could use either 'God!' or 'Damn!' on its own to show annoyance, disbelief or frustration. Nobody would be offended.

Just to add to John and Su.Ki.'s answers... A good guide for using colloquial language (slang, expletives, etc) is to listen for them first. That is, wait for the other speaker to use them first before you use them. Not much point in analysing the phrases - just follow the real context. A common mistake by learners is to use expletives to show they know "real" English, and the result is usually very strange or comical. I'm sure that when you hear non-Italians swear in Italian, it sounds cute or funny, or simply odd to you. :)

It is not polite, however it is rarely used as blesphemy. In very very rare cases, highly religious people may get annoyed by it however. In Britain, nobody would get too upset but i would only use it with friends. If you say instead: 'damn it's raining again' it is less likely to offend anybody :)

hi everyone, someone could explain me if the word "god damn" is a BLASPHEMY? Just to reinforce what others have said. It isn't British Slang, but maybe it is American Slang. From my point of view. Yes it is blasphemy. But note, that assumes that blasphemy exists. I struggle with the idea of a omnipotent god being upset by a few nasty words. I recently listened to a Frenchman chatting in English using bad words from the 1960s, hilarious. With your example I'm getting hints of a cartoon from the 1970s about a backwoods man deep in the heart of America.....