Makai Senki Disgaea: Revelations
Thanks first off to the minds behind Jim Breen's WWWJDIC and 漢字そのままDS楽引辞典, the two dictionaries I made excessive use of for this translation.
Overall, I have tried to keep the translation as literal as possible without making the English too stiff. In some cases, this means the sentence structure has been altered to reorder clauses, or to split them up to avoid run-on sentences. I have also translated the narration into past tense, since this is the most comfortable for English.
Nippon Ichi Translations
For consistency's sake, I've used some of Nippon Ichi's established translations, even if they aren't exactly literal.
The word Etna and the prinnies use to refer to Laharl is not Prince (王子 ouji), but 殿下 denka, which is more equivalent to the English "Your Highness," the appropriate title for a prince. His proper title, 陛下 heika, is occasionally used in this novel, and I've typically translated it as "Your Majesty" for emphasis, though in some instances I've translated it as "King."
I'm not sure how they decided on this particular translation. The prinnies almost always end their sentences with っす -ssu. This is a sort of compressed form of です desu, which is just the polite form of the verb "to be."
Defender of Earth
Gordon's title is more literally "Hero of Earth." What was translated as "Slayer of the Netherworld" is really just the same title but with "Netherworld" in place of "Earth." This, I've translated as "Defender of the Netherworld."
Laharl calls Flonne 愛マニア ai-mania, so it could also be translated as "Love Maniac."
These are actually written in English in the novel. That is, Giga Fire is written ギガファイア gigafaia. The only exception is Ice, which is written as "Cool" (クール kuuru).
All of the Disgaea characters speak in distinctly different ways. In the novel, dialogue is almost always in its own separate paragraph, and frequently the only indication as to who's speaking is the manner in which the line is said. Unfortunately the different speech patterns are very difficult to translate into English. I've done my best, but a lot is lost in translation, so here is a general idea of how the characters speak:
If you've heard his Japanese voice in the games or anime, he probably sounds rude and obnoxious even when you can't understand him. And he generally is. However, his speech patterns actually have more in common with characters like FMA's King Bradley than with your average mannerless male protagonist. He speaks like someone in a position of authority, who does not use respectful language towards others because he doesn't have to; he's at the top. He will also sometimes use phrases that are archaic by modern standards. One thing he does that's very distinctive is to refer to himself as ore-sama, elevating his own importance. Otherwise he never uses any suffixes, always referring to people by their names only (or simply as "you").
Etna's speech actually reflects more awareness of rank than her tone implies. When speaking to Laharl, she uses standard polite endings, while with most of the other characters she speaks casually. She refers to herself as atashi, but otherwise her language tends to be a little more masculine than feminine. When acting as Laharl's vassal in welcoming Yasurl and the others, she actually uses keigo, honorific language, which is about as polite as you can get. She will also use Laharl's proper title on occasion, depending on who she's speaking to.
Flonne's speech is the closest to standard polite Japanese out of any of the characters, though with some feminine endings. You could find some of the things she says in textbooks. She generally speaks to everyone with polite forms and refers to them all with -san. Children, she will refer to with -chan and use casual forms, which is also the accepted norm. She seems to use keigo for Lamington and refers to him as -sama. Laharl's mother also gets -sama, though not his father.
Baal uses casual masculine forms, but also sometimes speaks a bit like an old man. That is, he refers to himself either as washi or 我 wa, and his word choice and verb construction at times tend towards older Japanese.
Yasurl typically uses casual forms and feminine language. She sometimes addresses Laharl with polite language, but it comes off as even more insincere than Etna.
Vesuvio speaks like a very feminine woman.
Shas uses casual, very childish language with everyone, and calls everyone -chan. Her parents are the exception, both of whom get -sama.
Kira uses polite language similar to how Flonne speaks, but slightly more masculine, and since most of the characters are older than him, he only uses casual forms with Jane and Shas, or when talking to himself. He also manages to use Laharl's proper title. Evidently he's the younger twin, as he refers to Shas as 姉さん nee-san.
Gordon speaks like the defender-of-justice type hero you'd expect from his character. He refers to himself with the standard watashi, and uses casual, masculine forms.
Jennifer uses casual forms and feminine language. Additionally, although all the characters use English loan words, Jennifer uses some English that hasn't become common to the Japanese language.
She speaks like a little girl, similar to Shas but generally not quite so childish. Sometimes they can be hard to differentiate, though.
Thursday uses plain/casual forms with Gordon and Jennifer, but polite desu forms with Flonne and Laharl. All his speech is written in カタカナ katakana and kanji, which manages to make it look stilted and robotic. A similar effect is achieved in English by using all capitals.
Japanese has a ton of these and it can be very difficult to translate them. For example, when describing Yasurl, there's an onomatopoeia that essentially indicates an hourglass figure. A lot of the time there is no good English equivalent, so instead I've resorted to related adjectives or verbs. In other instances, the verb and the onomatopoeia both translate to essentially the same thing. E.g., こんこんとドアに叩いた is literally something like "He knocked on the door with a knock-knock." Consequently, in some cases I've collapsed the onomatopoeia-verb combination into one phrase.