The Charity Commission has reduced the amount it spends on its main Welsh translator by 72 per cent since 2009.
The commission paid Adral £29,870 in 2009, but this fell to £8,491 in 2018, according to figures released after a request made by Third Sector under the Freedom of Information Act.
Joe Saxton, co-founder of the voluntary sector research organisation nfpSynergy, said the figures highlighted the need for a charity regulator for Wales, a move he called for last month.
"It's really important that charities and individuals across Wales are able to access services from the commission in Welsh," said Saxton.
"It's disappointing to see that the expenditure on translation services has fallen so much over the past decade.
"For me, this reinforces the need for a separate charity regulator in Wales that is responsive to local needs, including language needs, to help the charity sector in Wales flourish."
Anna Nicholl, director of strategy and sector development at the Welsh Council for Voluntary Action, said: "The WCVA has raised with the commission our members’ concerns about timely and accessible Charity Commission services through the Welsh language.
"I hope this will be an opportunity to work together to explore how we address this in support of a vibrant culture and thriving Welsh language, one of the seven wellbeing goals under the Well-being of Future Generations (Wales) Act."
The commission's spending with Adral, an English-to-Welsh translation service, has fallen every year since 2009 except two.
This has coincided with severe cuts to the regulator's budget. Its budget was £29.3m at the turn of the decade, but this figure fell to £21.3m in 2014/15 and was further reduced to £20.3m the following year.
It was then frozen at this level until 2020.
A commission spokeswoman said it was "highly committed to meeting the needs of Welsh speakers and our obligations under the Welsh Language Act 2011".
She added: "Expenditure on translation services corresponds directly to business needs and varies depending on which projects were carried out in any given year.
"During the period 2008 to 2015, work was done on the commission’s website and the annual return, we produced proactive reports about the Welsh charity sector and we led engagement activities in Wales, all of which will have required translation work.
"Since that time, proactive work across both England and Wales has been scaled back due to significant reductions in our funding.
"All new services since 2017 are designed and go live bilingually, including the Apply to Register a Charity service, where charities are able to register online in Welsh, and the online annual returns, where charities can update their information entirely in Welsh."
The spokeswoman added that its casework, digital and telephony services, and trustee guidance were all available in Welsh.
She added that the commission used other translation companies and some translation was done in-house by Welsh-speaking staff.