When I typed "spicy natto" on Google Translate, I got this:
Here are two things that confused me:
1) why do we need な?
2) why not use 辛い?
Does anyone know?
Usually when 外来語 are used as adjectives, you should use な and use them just like you use Japanese adjectives. The only exception is when the whole phrase is a brand name or something, you can just useスパイシー納豆.
Now as to why Google chose スパイシー over からい, well there is a slight difference between the two. からい has more a nuance of something that burns the tongue and スパイシー is more about the taste to me, but this might differ from people.
And to be honest I don't think Google translate is that accurate so it probably chose スパイシー because it's easier...
1) Why な is used after スパイシー
when Japanese language imports a word from a foreign language, it almost always receives and encapsulates it as a noun regardless what the word's original function was in its source language. From there, the native speakers will attach ～な to use it as if it were an adjective.
This happens because, in Japanese speakers's mind, the meaning associated with the foreign word is registered as a concept, devoid of the original word's grammatical role, since the person doesn't speak its language. In fact, you are feeling strange to see this usage of attaching ～な after スパイシー probably because you're an native English speaker and in your mind 'spicy' is already registered as an adjective. But, in Japanese speaker who only takes the impression of this word "spicy" as to what kind of concept the word is about, devoid of its syntactical role in original language, perceives it as "spiciness" rather than 「spicy~ (some-noun-following)」. Therefore, to use this newly imported concept 'spiciness'-- a noun-like-word in his/her mind -- as an adjective, he/she should feel natural to add ～な after it to give it a role as an adjective.
2) Google translation is useful as mere a reference or a tool to find out what the word's rough meaning is; but, definitely not dependable as replacement of Japanese to English dictionary. It does better as English to Japanese translation tool, though. Also, it's true that there is difference in nuance between スパイシー and 辛い, but I suspect that this is just Google translation's short coming that is causing you the confusion, unless the spice used to make 納豆 taste spicy is originating from western cooking; which, even if so, Google translation has no way to detect from just two words!
Google: spicy -> スパイシー 辛い -> spicy Japandict: spicy -> スパイシー 辛い -> spicy dictionary services on Japan side 英和郎: spicy -> taste of spice 辛い -> hot as pepper Weblio: spicy -> taste of spice 辛い -> pungent, hot, acrid, spicy as in the case with curry goo: spicy -> taste of spice 辛い -> hot as if stinging the tongue, taste of spice, peppery Also, 「辛いラーメン」 but not 「スパイシーラーメン」
To Japanese locals, or those who have been eating Japanese food or other cousins' other than just e.g. American food, 辛い is a differing taste from spicy taste, but to those who have not, calling all such tastes as just "spicy" wouldn't seem to be a problem.