Does Lelouch vi Britannia speak with a Japanese accent?
This is a suspension-of-disbelief issue within Code Geass. Also pretty much every other anime. "Foreigners" speak better Japanese than they do English, like the two British girls from Kiniro Mosaic. It's rare that a "native" English or German speaker is used for such characters. 

Everyone, even the Britannian nobles, speak perfectly unaccented Japanese in Code Geass. Nothing in the anime or its manga adaptions suggests the Brits have any kind of "foreign accent". You can read this as a plot whole, but I would take it more as a concession to the format.

Like any other SciFi anime. Did people in Gundam go into space speaking perfect Japanese despite using strangely western names like Aznable and naming cities Von Braun and companies Anahiem? Did the Soviet Union and the United States suddenly start speaking Japanese in the Muv Luv series? If you watch scifi anime I think you need to suspend your disbelief and accept Japanese as the lingua franca. The original creators certainly did.