Depends. English is somewhat special in that it's the current lingua franca, so almost everyone has some exposure to it - and at the same time, it's a hodge-podge of words from different language families, so it's quite likely that an English speaker can pick out some words they understand from another language, if spoken slowly ("false friends" notwithstanding). So whether you're speaking English so someone who doesn't know it, or someone is speaking a language you don't know to you-the-English-speaker, there's stochastic benefit in slowing down.

It's the same phenomenon as within language families. I don't know Ukrainian, but I can pick up words that share etymology with Polish words, assuming they speak slowly enough for me to isolate individual words. Similarly, I don't know much German, but I can route individual words through English.