Najs is pronounced almost exactly the same as English 'nice', and the reason why is simple: it's a loan word, but has been adapted to fit Swedish spelling.
It can be used as a substitute for words like trevlig, fin, or skön, and can refer to how something looks or a general feeling, for example det är najs att dansa lite (it's nice/fun to have a bit of a dance). You'll also hear it as an exclamation, as in English, to express appreciation for something: najs!
Over recent years, Swedish has loaned plenty of words from English thanks to a growing level of English language proficiency and the wide availability of English books, films, songs and games.
This is in contrast to the situation a century or so ago: the Swedish Academy has existed since the 1700s, with the goal of promoting the “purity, strength, and sublimity of the Swedish language”, which for a long time meant discouraging the use of foreign words.
But today, there are lots of English loan words in Swedish. Sometimes, they'll keep their English spelling, especially if they're recent borrowings. But the more they are used, the more integrated into the Swedish language they become.
The first stage of this is usually adapting to Swedish grammar. For example, callcenter is usually written as one word in Swedish, which favours compound words. And commonly used verbs, like googla (to Google), snooza (not quite 'to snooze', but 'to press the snooze button on an alarm' or 'to temporarily delay') or attacha (to attach – an alternative to Swedish bifoga), usually take on the Swedish verb endings such as 'a' in the infinitive or basic form and 'ar' in the present tense: jag googlar (I'm googling).
Taking on Swedish spelling is the next stage of the loan word's integration, and it's more likely to happen the more common a word is. Nice/najs is quite a common word, and heavy usage probably sped up its Swedification.
As for why so many Swedes, especially young Stockholmers, prefer najs to the Swedish alternatives, it's probably down to a perceived high or cool status of the English language, or simply a desire to switch things up a bit.
Other words which have been adapted into Swedish in the same way include dejt ('date', but only in the romantic sense), tejp (tape) and tajt (tight). But the world 'live' in the sense of for example a 'live concert' (livekonsert) is still live so as not to confuse it with lajv, which refers to live action role-playing games.
And you may also be interested to learn that 'nice' wasn't always such a nice word. It actually started out as a negative term, which came from the Latin nescius meaning 'ignorant'.
That was the meaning when it first entered the English language in the 13th century, but over the years the meaning changed, first to 'timid', then to 'fussy/fastidious', then 'careful/precise' and eventually 'agreeable/kind'. If you've read this column before or know a bit about language, you'll be aware that it's common for a word's meaning to evolve over time, but such an extreme change as this is still unusual.
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Parken kommer att bli riktigt najs på sommaren
The park will be lovely in the summer