Most people in Spain are Spanish and speak Spanish. Most people in Italy are Italian and speak Italian. There are many countries that give their name, plus a suffix, to both a language, and an adjective for things from the country.
Of course there are lots of languages without countries, and plenty of countries that do not share a name with the language. But there are quite a few cases where the name of country, plus a suffix, gives a word that is both the name of the national language and also the adjective. To any fluent speaker of English, these are familiar words and cut-and-dried cases: the people from Norway are Norwegian and speak Norwegian, and the people from Sweden are Swedish and speak Swedish, but the people in Germany are not Germanish and do not speak Germanese. Let’s focus on the cases where there is a country, whose name (plus a suffix) is both the adjective, and the name of the language. (I’ll set aside the issue of the word for people from the country. We get three Italians who speak Italian, but three Englishmen – or women – who speak English, three Finns who speak Finnish, and three Spaniards who speak Spanish. We leave that for another day, but there’s more discussion here.)
There are four common suffixes: -ish, -ic, -ese and -ian (or sometimes -ean). We have:
Danish, English, Finnish, Polish, Spanish, Swedish, Turkish (and complicated cases like English, Northern Irish and Scottish, which I’ve disregarded because, technically, we are the United Kingdom, and also the Welsh might complain)
Icelandic and Arabic (not very many languages; I think the -ic suffix is a bit archaic, and goes with languages and landscapes that are dramatic, even operatic)
Burmese (or at least it used to be, not sure what to do about Myanmar), Chinese (one language or many? too complex to contemplate), Japanese, Maltese, Portuguese and Vietnamese
Albanian, Armenian, Bosnian, Bulgarian, Croatian, Estonian, Georgian, Hungarian, Italian, Korean, Latvian, Lithuanian, Macedonian, Norwegian, Persian (if you’ll permit a little historical flexibility), Romanian, Russian, Serbian, Ukrainian
There is also a group of -i cases, an Arabic suffix found in Somali (also in Iraqi, Israeli and Pakistani – but those aren’t languages) and also relating to the many official languages of India: Hindi, Bengali, Gujarati, Punjabi. Another set are akin to ‘bare names’: Czech, Malay, Slovak, Slovene, Tamil, Thai. And then some of Britain’s closest neighbours or allies are special cases: Dutch, French, German, Greek.
Is there a pattern to be found? One generalization is that country names ending in -ia tend to give language names and adjectives ending in -ian. There is a big cluster of them in Central and Eastern Europe. But watch out for Hungary! Many non-native speakers (and plenty of native speakers too) follow the rule in reverse and call the country ‘Hungaria’. Correct (with local variation) in Italian, Greek and Dutch: but in English, no ‘a’: it’s Hungary.

Adam: It seems, then, that there aren’t a lot of generalisations to be made. Another interesting point: it is odd that in English some people insist on using ‘the’ with the names of certain countries – Yemen and Lebanon spring to mind. No one who knows these countries says ‘the Yemen’ (in spite of ‘Salmon Fishing in the Yemen’), or ‘the Lebanon’, yet the use persists even amongst journalists and broadcasters – though it is not consistent and may just reflect personal preference. There are all kinds of historical reasons suggested on the internet, but no one produces a useful pattern that is not drowned out by so-called ‘exceptions’. And why, in contrast, don’t we say ‘the Iraq’ although the Arabic has the article ‘el’? Btw I know very little about this topic, as must be obvious…
Gill: there’s some evidence that the use of the definite article is in decline. I just checked BNC (1992) against more recent corpus data for use of (the) Ukraine and (the) Yemen. In all cases, it’s much more common to omit “the”. But in the BNC the ratios are roughly: Yemen : the Yemen = 7:1; Ukraine : the Ukraine = 2.5:1. And in our current data they’re more like: Yemen : the Yemen = 13:1; Ukraine : the Ukraine = 5:1. Ukraine has of course been all over the news recently, and my impression is that this has accelerated the trend away from “the” (though I have no actual evidence for this). In the case of plural names (typically groups of islands: the Philippines, the Seychelles, the Bahamas etc), the use seems more natural and there’s not much sign of the article being dropped.
Michael: Thanks for that – I was just checking ukWaC and agree that ‘the’ is not that frequent these days – at least for Yemen and Lebanon. I think the internet folk who discuss this are trying to find a logic behind it, but there isn’t any – each country has unique historical circumstances, and acquired an English name in different ways (e.g from written or spoken interaction), and at different times. It would also be interesting to compare what a country’s people(s) call their nation or city, versus what it is called in English (and other languages), e.g Marseille/Marseilles; la Suisse/Switzerland, al-Urdan/Jordan I’m sure there’s been lots written about this too.
Adam
I always believed tha people from Spain were spaniards. Please advise
Regards
Jose
Hi Jose. Yes, a person from Spain is a Spaniard, while the language and the nationality adjective are both ‘Spanish’. In fact as Adam says: (I’ll set aside the issue of the word for people from the country. We get three Italians who speak Italian, but three Englishmen – or women – who speak English, three Finns who speak Finnish, and three Spaniards who speak Spanish. We leave that for another day, but there’s more discussion here.)
A person from Spain is not Spanish but a Spaniard.
Thanks for your comment Edwin. If you read down to the second paragraph, you will see that the post mentions ‘Spaniard’ as the noun for a person from Spain. A person from Spain is Spanish (adjective) and is also a Spaniard (noun).
If we apply the rules described above, what, in your opinion would be the correct term for a person and language from the island of Jersey? In the indigenous language, the answer is Jèrriais, but we don’t seem to have an English equivalent. Should it be Jersese? Jersic? Jersian? Jersegian? Jersish?