Sardinian (Sardo) is a separate language, not a dialect of Italian. Road signs (town/city names) in Sardinia are often in both Sardo and Italian. You can think of it like the difference between Welsh/English.

> You can think of it like the difference between Welsh/English.

I don't think that's quite right: Welsh is a Gaelic language and English is Germanic, so they're from different families, while Sardinian & Italian are much more closely related.

Possibly more like Scots language(not Scottish Gaelic) and English

http://www.ayecan.com/read_scots/burns_museum.html

> Though his position in society wis ower laich for him tae hae the richt tae vote, Robert wis nanetheless passionate aboot politics and made his writin desk his ballot box. Scunnerin the weel-daein and pooerfu, hooanever could prove a fykie business.

> Robert Burns loved the lassies. He fell in love aften an wi ease, and wi mony weemun frae a wheen o backgrunds. The wey he treatit his lovers wis different ilka time, and ilka relationship affected Robert in its ain weys.

Sardinian and Italian are much much closer than Welsh and English. Speaking Italian, I can make out written Sardo kinda sorta ok, and I'm not a native Italian speaker. As an English speaker, I can't make heads or tales of Welsh.

Italy has only been a united country for about 150 years, so it hasn't been that long since Italian (as we know it today) is widely spoken. Previously, people tended to speak their local Romance language. See the intro and history sections of https://en.m.wikipedia.org/wiki/Italian language.