Bulb smart energy users have reported their meter displays appearing in Welsh - even though they are not in Wales.
Some of the energy firm's customers said their displays were showing the text "defnydd heddiw", meaning "usage today".
Fixing the problem involves navigating the menu, which also appears in Welsh.
Bulb said the problem occurred with one in every 200 of its smart meters and could be resolved in five steps.
The website, Money Saving Expert, said it had seen some complaints dating back several months.
One customer, James Tombs, who lives more than 100 miles from the Welsh border in West Sussex, said: "I don't live in Wales and don't know Welsh... I went on to the Bulb forums, found others with the same problem and followed directions to change [the] language.
"It has been fine since."
Another user, Brian Benjamin, said: "It just started out of the blue. I did email Bulb and they sent me a link on how to fix it."
How to change the display
- Press the ok button at the bottom of the in-home display, and then the right arrow button until you see 'Gosodiadau Wedi anfon yr'
- Press ok. This takes you to settings
- Press the right button until you see 'Dewis iaith Wedi anfon yr'. If this does not work, you may need to try the next option, 'Clirio Gosodidau Wedi anfon yr'
- Press ok
- Select English on the left, by pressing the left arrow, then press ok
A Bulb spokesman said: "In some rare cases, in-home displays switch to Welsh.
"While we think Welsh is a great language, we understand that in many cases people will want their display to be in English.
"Luckily the solution is just a few button presses away and language settings can be changed in less than 10 seconds."