>>57882802You’re an absolute nerd for even caring about this. But you’re also hilariously wrong so let’s me nerd for a bit and make fun of you.
First of all, ん is ん, romaji is not Japanese. No romaji is correct Japanese. The Japan government does not recognize any romaji system to be official Japanese. There is no correct romaji, all romaji are wrong. There is no reason to even argue about one romaji vs another because none of it is ever correct.
>B-but in practiceIn practice Japanese people use a mix or romanisation systems without following the rules of a particular system. It’s all completely random and rules don’t apply. In this weird mix of not following one particular system, both n and m can be randomly used.
If you care about actual pronunciation from actual Japanese native. ん can pronounced [n] or [m] or even [ŋ] depending on context. Notice how I use the International Phonetic Alphabet. n and m are not sounds. They’re letters. Any sound can be represented by any letter, it all depends on the language. (Or in the case of English, it makes absolutely no fucking sense).
Now, among the numerous romanisation system, you’re probably familiar with Hepburn and believe it’s the main one. (it’s not, the main one is nihon-shiki. Hepburn was invented for dirty EOP).
If you follow strictly Hepburn, in that romaji system they decided to transliterate ん as n. This Hepburn system is supposed (I really mean SUPPOSED) to mimic the English pronunciation of latin letters to match the Japanese pronunciation of kana. It’s obviously hilariously wrong because ん has multiple pronunciation and it’s always transliterated by n in that system. (that’s not the only kana that hepburn doesn’t match phonetically)
Now when you say "sempai" in an English sentence. You’re not writing Japanese, you’re writing English with a borrowed word. In that context you’re not using sempai as a Japanese word but as an English one. There is no reason to strictly follow Hepburn. Unless you always write Tokyo as "Toukyou" or Shojo manga as "Shoujo manga". At this point the word has been anglicized and the writing doesn’t matter as long as you understand the word and how it’s pronounced. Writing it Sempai highlight that in that context the ん is pronounced [m]. You can write it senpai, that’s fine. But it’s still pronounced [sɛmpaJ].
Now, you probably don’t want to make up new words and new variants that you’re the only one to use and follow the English speakers average usages. If everyone uses senpai and you’re the only one using sempai, you are against the common usage. But in practice both are used and indexed in dictionaries. Wiktionary has a page for sempai as a variant of senpai.
https://en.wiktionary.org/wiki/sempai 
