In plain English
FAR 52.214-34, Submission of Offers in the English Language, is a solicitation provision prescribed at FAR 14.201-6(w), most recently dated Apr 1991. The complete official text is reproduced below, verbatim, from GSA's published FAR source files.
Its text contains no sentence requiring insertion into subcontracts; see the flowdown section below for what that does and does not mean.
Does it flow down to subcontracts?
No flowdown mandate found in the clause text
This is a solicitation provision — it applies to offerors when submitting an offer, not to contract performance, so subcontract flowdown is generally not the operative question for it.
Where it's prescribed
As prescribed in 14.201-6(w), insert the following provision:
Prescribing reference: FAR 14.201-6(w).
The official text, verbatim
As prescribed in 14.201-6(w), insert the following provision:
Submission of Offers in the English Language (Apr 1991)
Offers submitted in response to this solicitation shall be in the English language. Offers received in other than English shall be rejected.
(End of provision)