You can ask "What do you say to the waiter?" or "How do you explain this to the waiter?" or even "How do you say this in English?" but you can't ask "How do you say to the waiter?"
It's fine to say "This coffee is lukewarm". Generally, it's clearer to use a word's literal meaning rather than a figurative one. Of course, that will only work if the coffee really is lukewarm. If it's fairly warm or fairly hot, just not hot enough, then it would an inappropriate choice. I would go with "This coffee's not hot" (well, I wouldn't say it at all because I don't drink coffee!)
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