How can I disguise my native language?

>> I will talk to them in language X and it turns out that they really cannot speak the language AT ALL, they STUDIED it or I will hear their speech and ow! it's like 'What in Sam Hill is that? What are they trying to say?" <<

Or it could be that they just are shy while speaking the language.

>> Google seems to work fine even there's some mistakes in the original spanish. <<

Maybe the author wrote in in English and then had it translated into Spanish.

When they answer in English, just say "es tut mir leid, ich verstehe kein Russisch"

Also, if they don't talk English to you, regardless of your accent, it's a sign you're fluent at it. You're fluent if you use idioms even a native doesn't know about or are creative enough to invent them on your own without sounding awkward.
A Serbian music teacher of mine used to talk with a heavy Slavic accent, but her Dutch was almost as good as mine. No one questions her fluency. People actually only do when your sentences are too simple.

Sucks for you you can't practice to become that fluent though.

<<You're fluent if you use idioms even a native doesn't know about >>

That's actually a sign that you are not fluent. Real native speakers know which idioms to use and which not to use. Using an obscure idiom will be a dead give away you're not a native.

Depends on the frequency you use them IMO. It's of course important to use at in the right context so people know its meaning. I get your point though. ^^