"How are you doing today?" Pretty hard to spin that into something fatal, except maybe through "today" aging you to death or time-looping you. But that's kind of silly.
Test log for Dr. ███████, 2/31/████
Test Subjects:D-555/1220
Native Language: English, American derivation
Word Spoken: "How are you doing today(inquisitive conjugation)" [See attached list for included morphemes
Result: No anomalous zone produced by vocalization. Subject reported a sensation of 'being watched', that continued for many minutes. As with D-425, subject reported smelling burning tires. Five minutes after initial vocalization, subject experienced an auditory hallucination, perceiving a voice directly to his left. Subject was unable to reproduce the sound, and the anomaly ended shortly thereafter.
You say "rain". This is not a very easily misinterpretable word. The effect starts out pleasant, in fact. And then, for inexplicable reasons, it turns into pure acid. That's not just "You shouldn't have eaten the plutonium", that's "The plutonium is trying to murder you". If the way it went bad were more related to the word, then it would be okay. But the only possible explanation for this is outright malice.
That assumes that the rain that a barely understood, reality bending language of which only five words have ever been uttered in a laboratory setting is anything like the rain that falls on you on a pleasant spring day. The hope was to create a magical language that was 'off'