In the Death Note 2015 series, some people sometimes speak English. For instance when Raye Penber and Bell talk and when Lind L Tailor comes out on TV, it's in English.
The Death Note anime of course acknowledges the existence of the English language, like in the first episode, when people with English names are written down in the Death Note and when Near says 'Mister' Aizawa, but a lot of English speaking people instead speak Japanese.
So this got me to thinking,
Are L, Mello, Near, Watari, Beyond Birthday et al Western people (See here for Wammy's House question) who just so happen to be fluent in or at least sufficiently speak and understand Japanese?
L once adopted the alias Ryuga Hideki. How would anyone think that a Western-looking person has a Japanese name? Is his cover story that he was adopted by a Japanese family or something?
Are they necessarily British just because Wammy's House is in England?
In general, is there anyway to know for sure what language is being spoken between a Western person and an Eastern person in the Death Note anime? For instance, when Light and L talk, are they talking in Japanese or English? How about when Near and Light talk?