In episode 1 of Time Travel Shoujo, it is established that Mari can communicate freely with local folk when she travels back in time. For example, when she travels back to 1600 and meets William Gilbert, she perceives Gilbert's speech as being in Japanese, and Gilbert perceives her speech as being in English. The audio itself is fully in Japanese during Mari's sojourn in Gilbert's time.
However, in episode 3, something curious happens. Mari has been sent back to the year 1752 and meets Benjamin Franklin. Again, she is able to understand him and vice versa. At around 14:37, though, we hear two background characters say the following in English audio:
GIRL: Rain, rain, go away! Come again on Saturday!
WOMAN: Good, Laura!
GIRL: Thank you very much!
To be clear, what I mean is that the viewer hears roughly 「レイン、レイン、ゴーア ウェイ。 カム アゲイン オン サターデイ。」 and so forth - the voice actor for the girl has a strong Japanese accent, but she's very clearly saying English words (ones that aren't loans into Japanese, in particular).
Now, why is that? Is there some sort of plot-related reason why Mari's "babel fish" didn't work on these two background characters? (The show pointedly addressed the idea of instantaneous translation during Mari's supplementary lessons earlier in episode 3, so I suspect this isn't just a goofup.)