Claudia has an indeterminable accent no matter which language he speaks, and it doesn't help that they can consciously change it. When speaking English, for example, she may make sounds that don't make sense to make for any kind of english speaker, though he has proven to be able to remove that if desired.
This accent could be from some ancient tongue humans forgot, a dead language, or a combination of everything. The only recurring quirk in his speech is that their 's' sounds are lazy and usually more similar to 'zh'