Language cookie module not working in Safari?
I think you should further verify/tune your configuration at Configuration -> Languages -> detection and selection: the order you mentioned (= 'URL, Cookie, User') seems a bit strange to me. I.e. it'd be interesting to see what results you get by specifying "Cookie" BEFORE "URL" in your case.

Because with your current order, "Cookie" will only be used to select your preferred language if from the "URL" it's impossible to decide about which language should be shown. Though "User" is probably fine to leave it as the last one.