Below I've presented findings for American English, British English, French, and German (Blue is "liberty", red is "equal". The years are fuzzy, but its from 1720 to 2000). A few things stand out to me:
1. "Liberty" is a fightin' word. The spikes for "liberty" associated with the French Revolution and the American Revolution are obvious. You also have a run-up to increased levels of "liberty" that peak at 1848 for Germany. France's radical mid-19th century maintains mentions of "liberty", in contrast to America which largely sets the term aside after the Constitution is ratified.
2. "Equal" emerges as a dominant term in America, not with the Revolution or the Constitution, but with the Jeffersonians. It maintains its position until present times. What's even more interesting is that if you substitute "freedom" for "liberty", then "freedom" surpasses "equal" in the late 1840s - at the end of the dominance of the Jeffersonians. I think there are two forces at work here: first, "freedom" was the word of choice in the abolitionist literature, rather than "liberty" - and after the war it stuck. Second, of course, is the Jeffersonian liberalism was a democratic liberalism rather than a strictly libertarian liberalism. It was liberalism. It's not like ideas of liberty were left out in the cold by any means. But it was a liberalism that emphasized both liberty and equality, and of course democratic self-government.
3. The British case is the only case where "equal" dominated "liberty" for the entire period (although Germany came close). This would likely look very different if the data stretched back to the 17th century. If "liberty" is truly a fightin' word, as I suggested in point #1, then the British had already had their fight.
It's dicey to draw too much from this data, but that's what I noticed - the point that "liberty" is associated with revolutionary breaks, while "equal" is more stable I think comes across very clearly, though. Is there anything else worth noting?