English speakers put the definite article "the" before the word "future" when they refer to the future (no pun intended). For example:
In the future, everyone will have access to clean water.
Does that imply that the English-speaking world believes in hard determinism, at least unconsciously? Does language in this case reflect the speakers' beliefs?
No, you are over interpreting the use of of the definite article and the use of informal natural language. Here the statement could be one of intent - "We intend that in the future ...", or one of wishful thinking - "We hope that in the future ...", or expectation - "We expect that it will be the case that in the future ..." and so on. In none of these cases is this the assertion "It is the case that in the future ...". That might be what the speaker intends but it is not the only meaning that the speaker could intend.
Informal natural language can be/often is ambiguous and need the context to clarify what is intended (and sometimes not even then). This is why quoting out of context is such a common rhetorical device, you can always/often find snippets of someone words/writing .. where they appear to contradict the meaning intended for the whole piece.
In its most literal sense, "The Future" refers only to events yet to happen - whatever they may be, and however they are decided, so it doesn't say anything about determinism.
Bu .. Isn't it more about anticipation ? If it's used in a sentence like "I have saved money for the future" then to an extent the person uttering that is taking control over future events. While that might not be hard determinism, it is an anticipation that one aspect to events to happen will be that there will be some cash lying about.
Actually I think it would be more fun if things were as you suggest. To avoid implying determinist, we could say things like "I'm saving for the futures" or "I am prepared for a future" which would probably raise a few eyebrows.
Or it could in some possible futures. Gulp.
Yes and No. Yes, we are all determinists when we refer to the future. No, the word the is not the issue. It is not the cause, but the effect. To take an example of Aristotle's, either there will be a sea battle tomorrow or there won't be. If there will be a sea battle tomorrow, this is the future. If there won't be a sea battle tomorrow, that is the future. Either way there is only one future. Therefore it is naturally called the future.
Quantum physics has several interpretations. One of them is the "many worlds interpretation," where every yes or no choice creates two universes, one where the answer was yes, one where it was no.
Even in this case, which very clearly admits multiple valid paths forward from the past through the present, we refer to "the future." However, "the future" now refers to "the set of all possible futures."
English users switch to "a future" when thinking about only a small portion of possible outcomes. It becomes "the future" once we no longer have a reason to believe there are any futures outside of it.
You've actually asked several questions in your OP.
If we define "the future" as meaning the entire probability set of multiple futures, then we've done away with any specific determinism (although we are still assuming there will be a future of some kind, which of course we do not really know). Thus I would answer your primary question with a simple "No" or at least "It's inconclusive" because it's easy for me to infer this definition of the "the future" from your example statement.
However, your latter question is what I think is most interesting. You asked:
Does language in this case reflect the speaker's beliefs?
To that inquiry, I respond "Yes". This is primarily because I think we are always revealing our beliefs when we speak (a pat answer I admit). However, when I think of the possible unconscious beliefs operating when someone makes a statement such as...
In the future, everyone will have access to clean water.
...I see some underlying considerations including:
These might actually be beliefs the majority of us humans share. I simply thought it might be helpful to look at some of the implications in your example scenario since you asked a question about beliefs.
