Zechariah 8:23 where it foretells 10 gentiles grabbing the skirt of a Jew and saying 'we will go with you."
Isaiah 45:14 which speaks of the unpaid laborers of Egypt serving as slaves to the Jews.
The WT refers to these prophecies in connection with the "great crowd" of Revelation 7. But what they never mention is that Paul alluded to a fulfillment of these two prophecies in the 1st century.
In the NWT at 1 Cor 14:25, at the very end of the verse, where it says "God is really among YOU," the reference column cites Zech 8:23 and Isa 45:14. The NWT is not being original in citing these two verses in connection with 1 Cor 14:23-25. You will probably find these two verses listed there in any Bible that has a scripture reference column. This is because it is commonly recognized that Paul is alluding to these two passages.
In the context of 1 Cor. 14:25, the "10 gentiles" and the "unpaid laborers" are the unbelievers ("an unbeliever or an ordinary person" - rNWT) who show an interest in Christianity. Once they become Christian they become part of the Jew(s)/Israel to which the unbelievers seek to associate with.
If the non-Jews in these two prophecies were referring to the "great crowd" who come out of a then distant future "great tribulation," then, there shouldn't be any 1st century application. Which explains why the WT never mentions any such application.
Concerning 1 Cor 14:25, the Commentary on the NT Use of the OT (G. K. Beale and D. A. Carson), p.742, says, in part:
Part of the solution may be found in the relationship between Paul's quotation of Isa 28:11-12 in 1Co 14:21 and his allusion to Isa 45:14 and Zech 8:23 in 1Co 14:25. While 1Co 14:22 mentions both believers and unbelievers, the illustrations in 1Co 14:23-25 focus exclusively on unbelievers and the way they respond to an encounter with a community that is speaking in tongues or one that is prophesying. . . His [Paul's] allusion to Isa 45:14, on the other hand, relates to a later phase in God's relationship with his people, one when even Gentiles would come to recognize the presence of God in the midst of his people and would worship him for who he really is. . .
. . . In Paul's scenario "it is the church - itself a predominately Gentile community - through which God will accomplish the eschatalogical conversion of outsiders," and the Corinthian believers have "stepped into the role originally assigned to Israel in Isaiah's eschatalogical drama." Hence the church is the fulfillment of Isaiah's [chap.45] prophecy about Israel.
It might also be noted that Paul alluded to these two prophecies (Zech 8:23 & Isa 45:14) and related them to "unbelievers and ordinary persons." This would hardly be an apt description of the "great crowd" who are described as 'washing their robes in the blood of the Lamb' and are 'rendering God sacred service in his temple day and night.' (Although, in contrast, the Pharisees saw everyday Israelites as, "this crowd who do not know the Law [who] are accursed people." - Jn 7:49)
This thread has a similar discussion of the topic of Zech 8:23. Linked here for cross reference purposes.
This thread is also topically related to this one, but it mainly focuses on the Memorial aspect of it.
See this thread for a discussion of the Great Crowd in relation to the Abrahamic and New Covenants.