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".
Surely the accent is there to indicate that the é isn't silent. If the accent wasn't there, Pokémon would be pronounced poke-mon, according to the rules of English. The accent is probably being phased out because 1) people were most likely leaving it out due to laziness and 2) it doesn't really matter because Pokémon is now an established brand and everyone knows how it's pronounced so the accent becomes unnecessary.
I would posit it has very little to do with pronunciation and quite a lot to do with trade mark and copy right laws.
Putting an accent on the 'e' magically transforms it from a "word" into a "uniquely identifiable brand name" which Nintendo can legally stop any one else from using. Haagen Das, Nescafe and many well known brands use non standard punctuation and spelling to make their brand name proprietary.
It indicates proper stress in pronunciation, just as an accent mark is intended to.
Natural pronunciation of words in English does not adapt well to Japanese loan words. The reason is stress patterns. For native English speakers, trochees, spondees, and dactyls are the most natural, especially with nouns. That said, we can safely assume that English speakers will emphasize the PO in POKEmon, but, as a foreign word with no established, entrenched pronunciation, the dactyllic stress pattern is often mistakenly applied - leading to a pronunciation something like POH-ka-mahn, for those who know the word to be three syllables, or POHK-mahn, for the more culturally challenged.
Japanese, unlike English, does not stress syllables. Rather, they have two tones, high and low, which serve an equivalent purpose. Tone variation can also be seen in English, for example, when we raise the pitch of our voices at the end of sentences to indicate questions (Japanese also do this). The tone pattern for Pokémon, in Japanese, is high-high-low-low (the final 'n' is a distinct syllable - unvoiced glottal stop). Illustratively, that could be written as POKEmon - the same result we get from forcing English speakers to accent the second syllable.