The short answer: use both, each where it belongs
Both names are correct. Caerdydd is the Welsh name of the city and Cardiff is its English name, and your customers use whichever belongs to the language they are writing in. A question asked in Welsh about a café in Abertawe and the same question asked in English about a café in Swansea are about one street and one counter, yet they use different words for the place.
Whatever answers either question, a search engine or an assistant, can only quote what your pages actually say. If your site says Swansea throughout and never Abertawe, the Welsh question names your town with a word your site does not contain. That is not a judgement on how well any system copes with Welsh. It is a plain fact about the page: the word is written there or it is not.
Using both does not mean writing every sentence twice, or turning an English page into a bilingual one. It means stating the second name once or twice, in the places where people and machines look for a location: the area you serve, your address, your page titles where it reads naturally, and your structured data.
Take the spelling from the official list
Welsh place names have standard forms, and there is one place to look them up. Comisiynydd y Gymraeg, the Welsh Language Commissioner, publishes a list of standardised Welsh place-names, and each entry gives the recommended Welsh form of a town or village beside its English form. Check every place your site names against it before the spelling goes onto your pages, your listings and the side of your van.
Some pairs are different words altogether. The list gives Caerdydd for Cardiff, Abertawe for Swansea, Caerfyrddin for Carmarthen and Hwlffordd for Haverfordwest. Others are the same in both languages: Caernarfon, Bangor and Aberystwyth each have one form for Welsh and English alike. You cannot work out either kind from the English name, which is why a place name is copied, not translated.
Copy the hyphens and accents as well. The list writes some names with hyphens, such as Pen-y-groes, and the hyphen is part of the spelling, there to show how the name is said. Then use that spelling everywhere you are listed.
The four places where a page should give both names
The area you serve, in a sentence. Somewhere on your site, usually the homepage or the footer, you say where you work. Write it as a sentence a person would say, and give the other language’s name once, in brackets: ‘We fit kitchens in Carmarthen (Caerfyrddin) and the villages around it.’ On the Welsh page the two names swap places.
Your address, in the language of the page. On the English page the town in your address reads Carmarthen; on the Welsh page it can read Caerfyrddin. The postcode, the building number and the phone number stay identical on both, and they are what show a reader that the two addresses belong to one business.
Page titles, where it reads naturally. Google asks for a page’s title to be in the same language as the page’s main content, says it may show other text when it is not, and advises against titles that repeat the same words. So the English title names Carmarthen and the Welsh title names Caerfyrddin. A title holding both names is the last place to add the second name, not the first.
Structured data, with both names stated. The schema.org property areaServed names the area where a service is provided, and it accepts a place rather than only a line of text. Give that place a name in the page’s language and the other language’s form as its alternateName, which schema.org defines as an alias. The town in your address goes in addressLocality, part of PostalAddress, in the page’s language.
- The area you serve, as a sentence, with the second name once
- Your address in each page’s language, postcode unchanged
- Each page title in its own language
areaServednaming the place, with analternateNameaddressLocalitymatching the address on the page
In Welsh, the name changes inside a sentence
Welsh has a step English does not. A place name mutates after certain words, so the city the list gives as Caerdydd is written yng Nghaerdydd for ‘in Cardiff’ and i Gaerdydd for ‘to Cardiff’, and Bangor becomes ym Mangor. A Welsh page written well can name its own town in every paragraph and never once use the form on the list.
That is correct Welsh, and it should stay. Writing ‘yn Caerdydd’ to keep the name whole reads as a mistake to any Welsh speaker who sees it. The answer is to make sure the plain form also appears where grammar leaves it alone: in the address, in a short label such as ‘Ardal: Caerdydd’, and in the structured data, where the name is always the form on the list.
People asking questions do the same. A full Welsh sentence typed to an assistant mutates the name just as your page does, ‘Oes trydanwr da yng Nghaerfyrddin?’, while a short search such as ‘trydanwr Caerfyrddin’ has no word to cause a mutation and leaves it plain. A Welsh page holding both forms has written down both things a Welsh speaker might type.
On an English page, Welsh names stay plain. ‘Based in Cardiff (Caerdydd)’ is right as it stands. English grammar does not mutate a Welsh name, so the brackets always hold the form on the list.
What not to do: lists of towns and copied pages
Do not paste every town you can think of, in both languages, into a footer. Google’s spam policies give blocks of text listing the cities and regions a page is trying to rank for as an example of keyword stuffing. One sentence naming the places you actually work does the same job honestly.
Do not build a Cardiff page and a Caerdydd page that say the same thing in English. The same policies describe pages aimed at particular cities or regions that funnel visitors to one page as doorway abuse. If your site has a Welsh version, the Welsh page naming Caerdydd and the English page naming Cardiff are one page in two languages, which is a different thing and the right one.
Frequently asked questions
Should an English-only website use Welsh place names?
Yes, once or twice: in brackets after the English name where you say which area you serve, and as the alternative name of that area in your structured data. It does not need a Welsh version of the site.
My town has the same name in Welsh and English. Is there anything to do?
Less than for most places. The Commissioner’s list gives Caernarfon, Bangor and Aberystwyth one form in both languages, so the name is already right in each. On a Welsh page the name can still mutate, as in yng Nghaernarfon or ym Mangor, so make sure the plain form appears in your address and structured data too.
Where can I check the correct Welsh spelling of a place name?
In the list of standardised Welsh place-names published by Comisiynydd y Gymraeg, the Welsh Language Commissioner, where each entry gives the recommended Welsh form beside the English one. Copy the spelling, hyphens included, and use it everywhere you are listed.
Do I need separate pages for Cardiff and Caerdydd?
Not in the same language. Two pages saying the same thing in English, one for each name, is the pattern Google’s spam policies describe as doorway pages. A Welsh page that names Caerdydd and an English page that names Cardiff are one page in two languages, and that is fine.
How do I see which place names my site uses now?
On each page that says where you work, use your browser’s find-in-page to search for every place you serve, once in Welsh and once in English. A place named in one language only is where the other name goes. For the structured data, search the page source for areaServed, or run the free assessment, which reads the page as a machine does.