Why is "Pokémon" written with an accent?

Is there a language-related reason why the word has an accent on the "é"?

The Japanese for Pokémon is "ポケモン" (pokemon), so it's not to represent a long vowel.

The mark in question is an acute accent mark and is absolutely intended to mimic the native Japanese pronunciation, which itself is based on the English words "pocket monster".

Because of English orthography, there is considerable ambiguity surrounding the pronunciation of the character "e". (Compare the way you pronounce the "e" in "pocket" with the way you pronounce the "e" in "peel" to see this for yourself.)

However, in Japanese, the orthography and pronunciation of "e" sounds presents no ambiguity. To accurately transcribe the "e" sound found in the English word "pocket", only the character ケ, which is pronounced as /kɛ/, can be used.

Therefore, to indicate the correct pronunciation in orthographically-complex, ambiguous English, the acute accent was used. The reason for this decision was because although no native English words use the acute accent mark, most English speakers, and many Japanese speakers, are familiar with the way in which an acute accent mark modifies pronunciation, due to the large number of existing French loanwords in both English and Japanese--entrée/アントレ, café/カフェ, and élite/エリート are familiar examples of this pattern.

Source: I am a Japanese speaker and have worked at Nintendo as a translator.

It's probably to indicate that the "e" is pronounced, not silent. The word "sake" (in the meaning of the Japanese rice wine) is sometimes spelt saké for that reason.

It's a stylistic choice that also emphasises that the "e" is pronounced. Think about how the word "Pokemon" looks devoid of two decades cultural osmosis. Given that "poke" is a slangy sexual term, the marketers did their due diligence and found a flashy looking way to keep the Japanese title.

In French loanwords, é (e accent aigu) is often pronounced as [eɪ] or [e] (as in fiancé, exposé, etc), so therefore Pokémon would likely indicate to most English speakers that the word is pronounced "po-kay-mawn".