A couple feel "discriminated against" after being told they cannot have their wedding ceremony entirely in Cornish.
Steph Norman and Aaron Willoughby were told by Cornwall Council their ceremony must be in English and Cornish.
The council said "declaratory and contracting words" can only be said in English or Welsh for a legally binding marriage.
Ms Norman said "I feel if everyone understands the language, it should be up to them what language they speak".
The couple contacted the council as part of their search for a registrar who speaks Cornish, when they were told their plans for the ceremony were not legal.
Ms Norman, 32, from St Anne near St Day, then researched the Marriage Act 1949 and found an exception only applied to Welsh speakers.
image captionCornwall Council told the couple their ceremony was not legally allowed to be entirely in Cornish
She said: "If you fluently speak Welsh then that's perfectly legal, but not Cornish.
"It feels like we've been a bit discriminated against, because it's not equal across the board."
The couple have learned a "good fair bit" of the language as it was "really important to our culture and our heritage," Ms Norman said.
"We just really wanted an authentic Cornish wedding and part of that would be incorporating the language.
"So we can a little bit, but not fully," she added.
Cornwall Council confirmed the "declaratory and contracting words" of a marriage ceremony must be conducted in English or Welsh to be legally binding.
It said: "We try to tailor each ceremony to the couple's wishes and Cornish phrases are often included in our ceremonies.
"Currently we don't have a fluent Cornish speaking registrar but we can accommodate a Cornish translator if one is provided by the couple."
Cornwall Council estimated in 2015 about 300 to 400 people spoke the language fluently and used it regularly, with about 5,000 more having some conversational ability.
Cornish language teacher Jenefer Lowe said she was "hopeful" that a couple like Steph and Aaron would be able to take their vows in Cornish in the future.
"What we really need for that to happen is for Cornish to get official status like Welsh and Gaelic," she said.
"That needs a critical mass of people using the language for it to be accepted.
"But because it's not official we don't have the ability to do legal proceedings at the moment."
She has seen an increasing interest in the language in Cornwall and from around the world and there had been "huge advances" in the last 20 years, especially with uptake from young people.
Teaching online in the pandemic had also increased the number of learners, she said.
"I have students in Australia, America and around the world, but we now need to translate that into more speakers and more resources.
"There has to come a time when it is used in all walks of life."