Canon language?

[The Infamous Cloaker]

Are the characters suppose to be speaking in Russian and we just hear it in English so that we can understand them?

I am mostly asking this because Sam comments on that he might not even know how to speak English anymore at a part in the game.

[Brovah]

The game is localized into English but it canonically takes place in Russia with everyone speaking Russian as their first language with the exception of Sam who was a US marine at the embassy in Moscow.

[Infected Voice]

Obviously canon is Russian and Sam speaks Russian too but with a very strong 'American' accent and uses English words from time to time.
In the English localization he can't say 'I might forget Russian after 20 years spent in Russia', it would be weird.

[Neynh]

Have you just discovered video games? :')

Original Version is Russian.. so yeah, if you play in English the game will be obviously in English...

[Pizza Basil]

Russian Sam even has a quote when asked about the occupying forces that he hopes he can still speak English even though everyone else is.