Traditionally, the west of the country as well as the capital Kiev has been Ukrainian-speaking, while the east and south - closer to Russia and including the explosive peninsula of Crimea - speak Russian.
But most Ukrainians are bilingual and switch naturally between languages depending on the situation.
Now after the incursion of Russian troops in Crimea and with a pro-European government in Kiev, the sound of words has never been more political.
"In politics, the use of language is a signal: 'with us or against us'," Ukrainian sociologist Iryna Bekeshkina told AFP.
After Russia's parliament gave Putin a green light to send troops into Ukraine, many Kiev residents have started speaking only Ukrainian as a sign of protest against the Kremlin's actions.
Members of ousted president Viktor Yanukovych's Regions Party usually speak Russian, while ex-prime minister and icon of the former opposition Yulia Tymoshenko, a Russophone, refuses to speak it even in interviews to Russian journalists.
The leader of the nationalist Svoboda (Freedom) party, Oleg Tyagnybok, went so far as to ask for an interpreter in an interview with a Russian channel at the height of the political upheaval in Kiev last month.
At the same time, lines are easily crossed when it suits the speaker.
Yanukovych came from the eastern Russian-speaking region of Donetsk but made efforts to master Ukrainian - the country's only national language - after his election in 2010.
Dmytro Yarosh, leader of the ultra-nationalist Pravy Sektor (Right Sector) movement, told AFP that he refused to speak Russian, but still announced his candidacy in May 25 presidential elections in English, Russian and Ukrainian.
Russian and Ukrainian are both eastern Slavic languages. But despite similarities in grammar and vocabulary and almost identical alphabets, they differ sharply in many ways and are not mutually intelligible.
Many Ukrainian-speakers consider the language to be closer to Polish than Russian.
While Russian was the official language of the Soviet Union, Ukrainian was also taught in schools here, so that every Ukrainian can read and write in that language even if they do not speak it at home.
Studies show just one percent of the population does not understand Ukrainian, while 30 percent do not speak it fluently, and the proportions are similar for Russian, said Bekeshkina.
Remarkably however, a majority of people in many eastern and southern regions - 67 percent in Dnipropetrovsk and 53.8 percent in Kharkiv - gave Ukrainian as their mother tongue in the last census from 2001.
In everyday life, many Ukrainians switch back and forth between Russian and Ukrainian - at the supermarket, the bank or when speaking with friends and family. Talk show guests on Ukrainian television often debate issues in both languages simultaneously.
One issue that sparked the recent language debate was a decision last month by the new government to drop a 2010 law that made Russian a second official language in parts of Ukraine and was passed with Yanukovych's support.
The law was eventually kept, but Moscow condemned the attempt to "restrict the humanitarian rights of Russians". Putin also used this reasoning to justify the use of military force by saying Russia had the right to "protect its interests and Russian-speaking populations" in Ukraine.
"The law wasn't working," said Bekeshkina, "but lifting it would have been stupid: the regions concerned would have interpreted this as 'the nationalists are imposing their rules'."
For former world boxing champion turned politician Vitali Klitschko, another presidential candidate, the language question was "artificial" and only used by politicians who have "run out of arguments".
"I'm a Russian-speaker... my mother is Russian, my father is Ukrainian, and I never got the impression that my rights were being trampled on as far as language is concerned."
Moscow may have jumped to the defence of Russian-speakers in Crimea and elsewhere in Ukraine, but for Myroslav Popovich, a Ukrainian philosopher, Russian was not the language most under threat.
Ukrainian is less widespread, few works of classical literature are translated into the language and there are few scientific textbooks in Ukrainian.
If the authorities make no effort to promote it, he warned that this was the language that was going to be suffering "discrimination".
COMMENTS (2)
Comments are moderated and generally will be posted if they are on-topic and not abusive.
For more information, please see our Comments FAQ
Language plays politics . Language has more influence than religion .