Dick Cole of Mebyon Kernow said the signs would bring economic benefits
Street signs in Cornwall are to be printed in English and Cornish as part of plans to promote the language.
Members of Cornwall Council have voted for future signs to be bilingual.
The council said the changeover would would not cost anything because signs would only be replaced if they were broken or needed repairing.
The council's cabinet also agreed that the use of the language must be considered by all its departments in their day-to-day work.
In reality, the council said this might mean document titles pages containing some Cornish.
The former Kerrier District Council, which was succeeded by Cornwall Council, pioneered the idea of street signs in Cornish as well as English. The new unitary authority has decided to adopt its predecessor's policy.
Councillor Graeme Hick said: "The sign manufacturers will put the Cornish underneath the English at absolutely no cost, so it's a win-win situation.
"We've got a legal obligation as a local authority to promote the language, and this is an ideal way of doing it."
According to best estimates, while several thousand people of the county's 500,000-strong population can read some Cornish, only about 300 are believed to be fluent.
Cornish is believed to have died out as a first language in 1777.
However, speakers protested after it was branded "extinct" in February by linguistic experts working on Atlas of the World's Languages in Danger, compiled by United Nations group Unesco.
The Cornish Language Partnership said the number of speakers had risen in the past 20 years.
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