Narendra Modi's landslide victory in the Indian general election last month marked a turning point in the country's politics – but it has also resulted in a radical change in the country's language of power.

India's home ministry has instructed civil servants in Delhi to use Hindi rather than English in all their communications on Facebook, Twitter and other social media, it emerged this week. Hindi is to also get priority on all government websites.

But the move has caused concern in regions where Hindi is not the main language, such as the southern state of Tamil Nadu, which was the setting for violent anti-Hindi protests in the mid-1960s.

Jayalalithaa Jayaram, the state's powerful chief minister – whose party is the third largest in the national parliament – wrote to Modi on Friday asking him to make English the official language.

Another prominent Tamil leader, M Karunanidhi, said: "The attempt to accord priority to Hindi, instead of treating equally all languages … will amount to treating non-Hindi speaking people as secondary citizens."

Politicians in Kashmir – India's only Muslim-majority state – also expressed concern.

Unlike his predecessor Manmohan Singh, Modi – a self-described Hindu nationalist – took his oath of office in Hindi, and not in India's other official language, English. And even though he can speak English, the leader of the Hindu nationalist Bharatiya Janata party (BJP) insisted on using Hindi during one-on-one meetings with visiting South Asian leaders. Though Pakistan's Nawaz Sharif could understand him, Sri Lanka's Mahinda Rajapakse needed an interpreter. The prime minister also gave his maiden speech in parliament in Hindi, a language he stuck to during his first meeting with the nation's top civil servants.Modi's ministers were quick to pick up the cue. So when China's foreign minister came calling, though India's new minister of external affairs Sushma Swaraj is proficient in English, she insisted on only using Hindi. (Much as the Chinese dignitary, also well-versed in English, stuck to Mandarin.)

Language was one of the trickiest challenges faced by Indian leaders after the departure of the British in 1947. It was not possible to give only one language priority in a multilingual polity. So a compromise was struck – English was given joint official language status with Hindi to assuage the fears of non-Hindi speakers. Twenty-two other Indian languages, including Tamil, were also recognised by the constitution, and are used as official languages in provincial administrations.

The expectation was that Hindi would gradually take over as the lingua franca of government, but English has remained well-entrenched in the corridors of power.

Modi's rise to power threatens to change all that. His Bharatiya Janata party has always been a strong proponent of Hindi. And many Hindu nationalists also harbour a visceral hatred of English, identified with India's colonial past. In fact, the country's English-speaking elite are often derisively called "Macaulay's Children", after the British administrator who introduced English-language education in India in 1835.

But the hasty manner in which the new government has sought to promote Hindi has got even proponents of the language worried.

"India's plurality is based on the plurality of languages and religions," said Hindi poet Ashok Vajpayi, a retired civil servant. "Language is an emotional issue. The cause of Hindi cannot be divorced from the cause of other Indian languages – all should be promoted. Hindi cannot possibly flourish at the cost of other languages."