How did “little tsu” become a lengthener?
On Wikipedia and elsewhere, Japan is written like so: Nippon ( にっぽん ).
What is that tsu doing in there? Why isn't it represented in the romaji rendering of the name?
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The つ character you're talking about is commonly referred to as "little つ" and looks like っ. This characters is not actually pronounced, but rather it means to take a small pause.
In the case of にっぽん, instead of pronouncing it as "nitsupon", you would be pronouncing it like "ni [small pause] pon" which is romanised as "nippon" which has a natural pause between the two 'p's.
But like sawa said in a comment, this is a very basic thing in Japanese and you would have learnt this from a textbook's first chapter.
For a more complete explanation though, you can visit the wikipedia page on it.
On Wikipedia and elsewhere, Japan is written like so: Nippon ( にっぽん ). What is that tsu doing in there?
The chiisai-tsu (small tsu) should be covered by any basic hiragana book, a good alternative is wikipedia's hiragana article. From the writing section:
A small tsu っ, called a sokuon, indicates that the following consonant is geminated (doubled). For example, compare さか saka "hill" with さっか sakka "author". It also sometimes appears at the end of utterances, where it denotes a glottal stop, as in いてっ! ([iteʔ] Ouch!). However, it cannot be used to double the na, ni, nu, ne, no syllables' consonants – to double them, the singular n (ん) is added in front of the syllable.
"Doubled consonant" is probably not the most straightforward way to describe it, but in general you can think of a sokuon introducing a slight pause before continuing with the next kana sound.
Why isn't it represented in the romaji rendering of the name?
It actually is, that's what the doubled 'pp' represents in that particular romaji system. The various romaji systems are often contradictory with each other, but most of them use this doubling-consonant system to represent a っ.