Actually I think if you live in the UK you can spell a name a foreign way and pronounce it how it looks in English.
I say this as someone with a German name that no one pronounces in the German way, married to a Frenchman whose name no one (not even people who can speak French) pronounces in the French way, and with a DS with a French name that no one pronounces in the French way.
Before I had DS all English friends and family went on and on about how it would be difficult to get people to pronounce a French name for DS - we never expected English people to pronounce his French name in any way other than the English way, so we specifically picked one that we didn't dislike the English pronunciation of. In the same way that if we'd given him an English name we wouldn't have expected our French and German family and friends to say it in the English way.
I think it's something that in theory no one likes, but in practise it's what everyone does.
Sorry to ramble on - but the short version is, in the UK Tintin is not pronounced in a French accent, so I don't think Valentin would need to be either.
But seeing as it's Valentine you're after it's all a bit irrelevant anyway 