Is a bad actor harder to detect when you don't speak the language being spoken?

No! Here is the amazing thing: Acting is not what you say, but how you say it.

Granted, something might be lost in translation in the instance of extremely different cultural norms (humor), but usually the cues from character responses and the phrasing and blocking of the picture indicate what the response should be. Actors either consistently make strong choices in context of scenes and perform those choices clearly, or they don't. Poor acting, or mediocre acting that is covered by virtue of production quality, is usually obvious in any language. (Caveat, classically trained actor + screenwriter/director.)

As for subtitles, I say the same. When I saw HERO and CROUCHING TIGER, I immediately wanted to read the screenplays to see how the images had been accomplished on the page because the writing was solid. It was clear to me that LET THE RIGHT ONE IN had a solid script. Generally it's impossible to make a great film from a poor screenplay.

I avoid dubbed versions at all costs, with exception to anime. Some directors (Peter Jackson/LOTR) are hands on with dubbing/dub actor casting, which helps, but for the most part I consider dubbing a loss to the original vision, even more so than an imperfect subtitle translation.