How does "It rain tomorrow" not indicate a concept of the future?
For that matter, how does "It rain tomorrow" not indicate a phrase in the original language that might as well have been translated into English as "It will rain tomorrow"?
It does indicate a concept of future. What it doesn't do is use a different tense—the key verb is the same as when talking about the present. It's a subtle distinction, but he is arguing that this lack of differentiation in verbs has an impact on internalizing future events.
And while you very well could make the translation from whatever the original was to "It will rain tomorrow," it might not be a literal one that preserves the original tenses, which would obscure exactly what he was trying to point out.
[does happen, will happen], [go, will go] - notice that the active verb doesn't change from present to future, rather there is an auxiliary added to mark the tense. Contrast to French where you have [vois/see, verrai/will see] and the verb itself is modified.
Right, English doesn't have a future tense (in the sense of tense as marked by inflection). So what's the contrast of note? That the one sentence marks the future by use of a modal auxiliary (in addition to the explicit "tomorrow"), while the other marks the future only by explicit description of the time "tomorrow"? Mark me as highly skeptical that this is of any significance...
I'm saying it's a strange example to say that some languages do or don't have a grammaticalization of time, and then give two supposedly contrasting grammars as an example of each, when actually neither of them really has a future tense.
For that matter, how does "It rain tomorrow" not indicate a phrase in the original language that might as well have been translated into English as "It will rain tomorrow"?