Ukraine's Russian speakers, concentrated in the east and the south where the law applied, viewed the action as vengeful.
In Moscow, Russia's leaders saw an opportunity.
Ukraine's new rulers took just five days to reverse course to once again allow the use of Russian in some schools, courts and other state institutions.
Shocked by the outcry it caused, Ukraine's acting president had refused to sign the legislation.
But those five days were enough for Russian President Vladimir Putin to set in motion a chain of events that have undermined Kiev's pro Western government and drawn large areas of the country back into Russia's orbit, abetted by a divided West.
While Putin has presented separatist violence in eastern Ukraine as spontaneous, interviews with Ukrainian politicians and security sources with knowledge of Russian thinking suggest months of detailed planning by Moscow.
A key plank of Russia's plan, they say, was to deepen splits in a country that has struggled to form an identity since it emerged from the Soviet Union in 1991.
To that end, Russia sought to exploit its connections to Ukrainian business, youth groups, the church, politicians and criminal networks.
The sources point to a paper from June 2013, described as a Kremlin consultation document by the Ukrainian newspaper Dzerkalo Tyzhnia and first made public in August that year.
It sets out Moscow's fear of losing influence in Ukraine and its desire to draw its neighbor into an economic union.
The Kremlin declined to comment on the document, entitled "On the complex of measures to involve Ukraine in the Eurasian integration process", and Russian officials have previously written it off as a "provocation" by pro-Western politicians in Ukraine.
Bearing no signature or stamp, it is hard to trace its provenance, but a former security source in Ukraine corroborated its contents.
He said he was present during conversations about the document involving officials in Ukraine with close connections to Moscow.
Like others interviewed for this article he declined to be identified because of political sensitivities.
The document indicates that as far back as early 2013 Russia was nervous about Ukraine.
Yanukovich's rule was widely seen as corrupt and the Kremlin was worried the president's unpopularity could harm Putin's plan to create a Russian-led "Eurasian" economic union to reunite part of the former Soviet Union.
Many Ukrainians believed Yanukovich was a Kremlin puppet, according to the document.
Moscow was worried it would lose all influence in a new Ukraine if Yanukovich and his Party of the Regions were toppled.
"This aggravates the threat of a seizure of power by forces hostile to the Russian Federation," the document said.
"As the Party of the Regions has suppressed any independent pro-Russian movement, the collapse of the Yanukovich regime would leave us in a "scorched earth" situation, without any influential political forces on which we could rely."
It said Russia should apply pressure to oligarchs who enjoyed preferential trade with Russia but at the same time publicly criticized Putin's plan to create a Russian-led economic union.
A month after the report was written, Russian Prime Minister Dmitry Medvedev decided to scrap quotas for steel pipe supplies, hitting at least one prominent oligarch.
Russian pipe makers had taken issue with cheap Ukrainian pipe imports.
In the same month, Russia's consumer watchdog banned imports of sweets from the Roshen factory belonging to Ukrainian billionaire Petro Poroshenko, now front runner in a presidential election due on May 25.
The watchdog cited health concerns for the ban, saying a carcinogenic substance had been found in Roshen's chocolate.
The document pinpointed one political movement in Ukraine that could help influence opinion, Ukrainian Choice led by Viktor Medvedchuk, a one-time adviser to Ukraine's former President Leonid Kuchma.
Putin is godfather to one of Medvedchuk's children.
On Ukrainian Choice's website, Medvedchuk is sometimes critical of the roles of the United States and European Union in the crisis that has followed Yanukovich's fall and Russia's annexation of the Crimea region.
He denies being pro-Russian.
In a statement to Reuters, he said Ukrainian Choice was pro-democracy.
One of its founding principles was the "decentralization of power followed by the transition to a federal structure, while maintaining the territorial integrity and unity of Ukraine".
Russia too has spoken in favor of a "new federal constitution" for Ukraine, a system which would strengthen regional governors, possibly allowing Moscow to retain its influence in Ukraine's industrial east.
In response to questions from Reuters, Medvedchuk denied he was a go-between for Russia in Ukraine.
He said he always had Ukraine's interests at heart and had nothing to do with the document.
"At this level all politicians are independent and act on the basis of their understanding of the public interest," he said.
Even so, a former intelligence source, who was present during conversations involving Ukrainian officials with close contacts to Moscow, said Russia had hoped Medvedchuk would become the savior when Yanukovich suppressed what it saw as inevitable protests.
But the protests came sooner than Russia expected and Moscow's plans changed.
In November 2013 thousands of Ukrainians took to the streets angered by Yanukovich's decision to spurn closer ties with the European Union in favor of Moscow.
At this time, according to two sources in Ukraine's political and security apparatus, two of Putin's close allies took charge of "Project Ukraine", designed to spread the message that many Ukrainians would lose out if the country looked West.
Vladislav Surkov, a Putin aide, cultivated ties in Crimea, and Sergei Ivanov, Putin's chief of staff, took over Ukrainian operations, the sources said.
The Kremlin declined to comment.
Surkov and Ivanov did not respond to a request for comment.
NO NEED TO SET FOOT
With Russia's interests entrenched in Ukraine, many of those interviewed doubted whether Russia, which has massed troops at the border, needed to cross into Ukrainian territory to fuel the uprising in its east, where two regions have voted for self-rule.
The chaos in eastern Ukraine may already have achieved many of Putin's aims.
The Kremlin denies playing any role in the uprising.
Mykola Malomuzh, director of Ukraine's foreign intelligence service for five years until 2010, said Russia's target now was the presidential election - to have it postponed or make it impossible to be seen as legitimate.
"Putin has a network of his own among the special services, pro-Russian organizations, and the old regime which wields incredible influence through the mafia-type organizations which dominate the economy here," said Malomuzh, who is also running for president on May 25.
He suggested pro-Russian forces were already trying to influence the front runners in the election or lobby for its delay, although this could not be independently confirmed.
"If there's no legitimate leader who the people believe in, with whom to hold talks .... there will be no leader enough in control to move towards Europe," he said.
Source: Google News