The word translated "language" is שָׂפָהsapha. This word appears to have a semantic range along the lines of "termination → shore → lip → language". This last step is analogous to how European languages use "tongue" to stand for a language.
The word translated "speech" is דְבָרִים dvarim, actually the plural of דָּבָרdavar. Usually this means "word" or "thing". The "one" is also plural, suggesting an adjectival sense, something like "unified".
So what's the distinction between "one lip" and "one [set of] words"?
The Cambridge Bible for Schools and College says that this is merely "an expressive phrase", which I take to be a reference to the common Hebrew poetic technique of duplication. We can't rule out that possibility, but even so, the duplicated terms aren't always identical in meaning.
In fact, several older commentators see the difference as significant. They tend to see "lip" as a reference to pronunciation and "words" as a reference to the vocabulary:
...both the pronunciation and the vocabulary were identical. (Ellicott)
...perhaps it was pronounced by the lip and other instruments of speech in the same way; so that there was no difficulty in understanding one another ... all spoke the same language and used the same words. (Gill)
Of one language. Literally, of one lip, i.e. one articulation, or one way of pronouncing their vocables. And of one speech. Literally, one (kind of) words, i.e. the matter as well as the form of human speech was the same. (Pulpit)
As far as pronunciation goes, it might be worth noting that the Hebrew writers certainly did think about that quality of language, e.g. in the well-known "Shibboleth" episode in Judges 12:5-6.
One more commentator seems to read "lip" as a reference to both pronunciation and grammar:
The two terms are not synonymous or parallel ... "One stock of words," then, we conceive, naturally indicates the matter, the substance, or material of language. ... The term "lip," which is properly one of the organs of articulation, is, on the other hand, used to denote the form, that is, the manner, of speaking; the mode of using and connecting the matter of speech; the system of laws by which the inflections and derivations of a language are conducted. (Barnes)
More recent commentaries seem not to be interested in the difference between these two words, perhaps a reflection of a trend away from literalism or making much of stylistic minutiæ.
Appendix on linguistic diversity
In your comment you ask about a possible connection between "lip" as boundary and the idea of having one language as a limitation or boundary. I wouldn't endorse that as a primary reading, but it could have been present in their minds as wordplay on having one boundary — being one nation. In support of this, I note that the verse literally reads that all the earth "was", not "had", one sapha. (Also as wordplay, perhaps all the world also "was" one set of undiversified dvarim "things"?)
However, your idea about it being a limitation is explored by David Smith and Barbara Carvill in a wonderful book on teaching foreign languages through a Christian lens: The Gift of the Stranger. They point out that the God of Genesis is a big fan of diversity in plant and animal life, and tells humanity to "be fruitful and multiply and fill the earth" (Gen 1:28). This is all part of His plan for creation before the Fall. But at Babel, they argue, humanity disobeyed this command: "Let us build ourselves a city ... otherwise we will be scattered over the face of the whole earth" (Gen 11:1).
Because of that, Smith and Carvill disagree with the tradition of commentators who see linguistic diversity as a punishment, a situation somehow worse than having a single universal language. (This is often tied to old ideas about Hebrew being the perfect original divine language — but then why does God ask Adam to creatively come up with names for the animals?)
On the contrary, linguistic diversity happens naturally when people spread out. At Babel, God had to jumpstart the process and give humanity a kick by making it hard to stay in one place.
(If linguistic diversity interests you, it's a good book to read. They make the case that teaching, learning, and sharing other people's languages is a way to show Christian hospitality.)
The great medieval biblical commentator/grammarian, Abraham ibn Ezra explains that the word "saphah" refers to the language while the word "devarim" refers to the sophistication of speech. That is to say that though normally there is a broad range of sophistication of language usage/vocabulary between the wise men and the fools, even when speaking the same language, the Bible is pointing out that the people constructing the tower all spoke the same language and additionally they spoke it on the same level.
Genesis (11:1) “Everyone on earth had the same language (sa-pa) and the same words (de-bar-im)”. (11:6) And the LORD said: ‘Behold, they are one people, and they have all one language (sa-pa)). This is an iteration of the same thought in verse 1 except the phrase “they had... the same words” is now expressed as “they are one”. De-bar-im is missing from verse 6. (11:7) Come, let us go down, and there confound their language(sa-pa), that they may not understand one another’s speech(sa-pa). Again de-bar-im is missing and sa-pa is used twice.
It seems to be saying that people not only used the same language but they had a universal understanding. The writer seems to be expressing that while God changed their (sa-pa) language, he did not change (their understanding) what they were trying to say (de-bar-im).
I would add from an article I just wrote, part of which is the same issue:
" Genesis 11:1 And the wholeH3605 earthH776 wasH1961 of oneH259 language,H8193 and of oneH259 speech.H1697 ( with Strongs numbers)
The point of contention and to the topic at hand? Those are two different words with two different creating effects in the mind of the new creature in Jesus Christ in whom dwells the Word/Speech of God. One means lip/mouth and the other essentially what is being said ..Doctrine/matters, reports.. ( this an edit from earlier ). When all you can see is non-creating speech in print, that means that that is all you will ever look for and the common sense of it is all the wisdom you will ever have until you hear God.
It appears, if you read it and you are yourself in non-creating language, ( and of course don’t know that you are inside anything but imagine your language to be the universal language ) then Genesis 11: 1 appears to make God out to be saying the same thing twice:
KJV: ( without Strongs numbers )
Genesis 11:1 And the whole earth was of one language, and of one speech.
..not just one lip/pronunciation but the same speech as a whole living spirit ..non-creating speech/anti-Christ. We know it was non-creating speech because the definition of the Fall of Man was the switching of Speaks in Eve and the subsequent “hearkening to the voice of his wife” by Adam ..not simply because she said something but because she was speaking in dragon-speak/non-creating/Word-of-God-hating speech, ..and she didn’t know it. The serpents speaking event was sorcery by which he implanted in her flesh his own non-creating speech in her mind word for word for the pre-incarnate Word of God which was our native speech in Adam. She went from fluent in pre-Incarnate-Christ to fluent in anti-Christ and never saw it coming or thought anything had happened except her “opinion” of what to do with the fruit of the tree of knowledge of good and evil and what to do with speech itself suddenly altered. She never thought about her former speech in that instant, but only former supposedly evil restraint.
1Timothy 2:14 And Adam was not deceived, but the woman being deceived was in the transgression.
So by the time our ancestors got to the Plain of Shinar, they were ONLY fluent in anti-Christ. And that spirit was in all the people, (Romans 8:20 For the creature was made subject to vanity, not willingly, but by reason of him who hath subjected the same in hope, ). What God confounded was NOT Himself as Creating Word/Speech/Christ but the non-creating speech/anti-Christ. He confounded it by mixing up how it is pronounced, the grammar, the thought patterns ..not just a substitution cypher. God confounded their speech so that their lip/language would be changed and that spirit itself could no longer take advantage of all the people, at least not at the same time. He gave their souls a safe haven even inside anti-Christ for those whom He intended to save, even as He did later in the Babylonian captivity of Israel and even later when He made a place for them in the foreign countries. He stuck some good in the evil, but only for His people and did not empower the evil thereby.
But the points here is that they most decidedly did NOT speak pre-Incarnate Christ and God is not saying the same thing twice in Genesis11:1.
God differentiates the Word that He IS by prefacing what happened with the vital truth that their language and speech was one and He subsequently divided their language ( way of pronouncing and thinking language ) and thus confounded their language so that they could not understand the spirit of the anti-Christ whom IS their speech in their former compatriots in pronunciation. God most certainly did not confound Himself as Speech but by His Creating Speech that He IS confounded anti-Christ to protect the people.
Genesis 11:7 Go to, let us go down, and there confound their language, that they may not understand one another’s speech.
“Language” and “speech” are very plainly here two different words with two different creating effects in Christ. Only those outside of Christ would think “God is saying essentially the same thing twice in a will-neutral medium by which we process information so that our free will remains intact at all times.”
Outright lies in Ancient texts of anti-Christ masquerading as Holy Script ( comment mine):
Targum of Onkelos: “In the holy language spake they, that by which the world had been created at the beginning. And it was while they were journeying from the east that they found a plain in the land of Bavel, and dwelt there.”
Jerusalem targum: ” …for they spake the holy language by which the world was created at the beginning: while their hearts erred afterwards from the Word of Him who spake, and the world was, at the beginning; and they found a plain in the land of Pontos and dwelt there.”
You see that? A direct claim that God confounded His Own Self as Speech. “for they spoke the holy language by which the world was created”.. given that in John 1:1 God plainly declares as an act of His creating Word/Speech that He IS His Own Speech. He doesn't "use language" as a tool or medium outside of Himself.
John 1:1 In the beginning was the Word, and the Word was with God, and the Word was God. The same was in the beginning with God. All things were made by him; and without him was not any thing made that was made. In him was life; and the life was the light of men. And the light shineth in darkness; and the darkness comprehended it not.
Below is a reference on the two different words God Speaks in Genesis 11:1.
Note sâphâh (שָׂפָה) ( language) is feminine singular, while debârîym (דְּבָרִים) ( speech) is masculine plural ( as in “Mark 5:9 and he was questioning him, What is thy name?' and he answered, saying, Legion is my name, because we are many;’)
sâphâh (שָׂפָה) [pronounced saw-FAWH] lip, tongue; words, speech; dialect, language; edge, border [or, lip] [of something], shore feminine singular noun Strong’s #8193 BDB #973
This is the first occurrence of this word in Scripture; this was not the word used in the previous chapter.
debârîym (דְּבָרִים) [pronounced dawb-vawr-EEM] words, sayings, doctrines, commands; things, matters, reports masculine plural noun Strong’s #1697 BDB #182
Although this seems nearly impossible to me, this appears to be the first occurrence of this word in Scripture.
My article is much longer and bit off topic for the rest of it, but can be at 2equal5.com