The Japanese Language is considered an extremely "complicated" language to an English speaker's ear. While certain concepts are simplified (very few real plurals, for instance), the grammar is switched around, and both the words and wording are often grounded in concepts that are either different or entirely external to the English language. And let's not even start getting into things like etiquette and connotation.
Unfortunately, some of those concepts are required to understand the full depth of the original script in Japanese-language programming. Such issues are why translators and fansubbers have a "rough" and "not-often appreciated" job on their hands.
Among the various aspects of the language that may or may not have relevance to the foreign viewer are the following:
Unsound Effect: Some Japanese onomatopoeias are for things like objects or people being sparkling clean.
Some less prevalent, but useful concepts to know:
Japanese verbs take two basic tenses, past and non-past. There is no future tense of the verb, so future actions either rely on conversational context or can be specified with time-related words.
Japanese has five vowels, of which four are present in English as the tense 'a', 'i', 'o', and 'e'. The fifth vowel is an unrounded 'u' which forms at the same place in the mouth as English 'u', except that the lips form a flatter shape. Furthermore, that unrounded 'u' is subject to devoicing (which sounds to English speakers like becoming partially, but not completely, silent) between unvoiced consonants such as p, t, and k. The good news is that this is much easier to actually pronounce than to write about; it's easier for foreign speakers to adjust to than the tone contour systems of Mandarin and Cantonese.
Japanese has a simpler syllable structure compared to English. This is exacerbated because words borrowed from Chinese, which include most of the complicated terms (think Latin and Greek with English), use only a subset of the sounds of Japanese, and the tones that would distinguish them in Chinese are lost. This makes homonyms more prevalent; humour based on this, such as puns and malapropisms, tends to be easier. This is especially true when noting that the kanji for words that sound the same can make the difference clear. A good example would be the title for Ai Yori Aoshi, which literally translates as "bluer than indigo" but is simultaneously a pun on ai, the word for love (an equivalent meaning would be "true blue love") and an allusion to the main character Aoi-chan.
There are no plurals in Japanese - if someone was talking about "kimono", there would be no way of knowing if it was one garment, two garments, ten, or ten thousand, or even All of Them. The closest they have is the term "-tachi", which can be added both to regular nouns and proper nouns, and means "and all the rest" (when used for proper nouns this is often translated as e.g. "John and his friends" or "John and his party"). There is also no way to indicate a group of X's in particular; "kimono-tachi" in Japanese could mean multiple kimonos or it could mean a kimono and a bunch of other clothing sitting next to itnote keep in mind that this is an example - "tachi" is typically used to denote living things, and using it with "kimono" will sound like a cute anthropomorphism.
That said, if it is important to distinguish between singular and plural, it is common to simply add the number, i.e. "one kimono" or "ten kimono" to specify what you want. Unfortunately, this brings up a whole new complication since Japanese people were not content with just one set of numbers. Instead, each category of objects has its own "counter" word (like group, flock, fleet, etc. but for everything) that must be used with the actual number, and you have to learn every one. Examples include -nin for people (3 people = sannin, 6 people = rokunin, though 1 person = hitori and 2 people = futari), -dai for machines and vehicles, -hon/-bon/-ppon for long, cylindrical things, -mai for flat things, -satsu for books and magazines, etc.
And before the point gets lost - "tachi" is not the only modifier of this sort, just the one most likely to be used in regular conversation.
Japanese people describe things like emotions and preferences differently than in other languages. In Japan, it is widely believed that you don't have direct knowledge of what other people are really thinking (and it's very presumptuous to assume otherwise), and so it is uncommon to describe other people's thoughts directly, such as "He likes ice cream" or "She's angry". Instead, it's far more common to see things like "I heard that he likes ice cream" or "It seems like/It appears to be the case that she is angry" or "She is showing signs of wanting to go to the park." Related to this, it is important to distinguish between information you know firsthand and information you've heard from another source.
In Japanese, the pitch of homophone pronunciation can alter the meaning of the word very dramatically; there are comparatively few words in English where this is true; for example, "REcord (as in a high score, or a disc that plays music}" and "reCORD (as in what you do to get your music on a REcord)".
Foreign speakers need to be careful with this, as the homophone pronunciation can vary between regions. Ame, for example, can mean either rain or (hard) candy, depending on which syllable takes the higher pitch. However, the Kanto regional accent (the Tokyo accent) uses different pitch-accent than the Kansai Regional Accent. Context is (usually) the important key here.
However, not having a good grasp of the pitch system won't lead to a faux pas - 'it's raining candy outside'note incidentally, just saying "it's raining" in Japanese would sound odd, along the lines of "there's rain outside" in English - technically true, but eyebrow-raising is a highly unlikely phrase in real life, after all, and there aren't many common homonyms in Japanese that are so closely related in meaning as "Record". It will just make you sound like a foreigner with an accent.
For the most part, if you can pronounce Italian or Spanish properly, you'll have a really easy time pronouncing Japanese. They have most of the same sounds, especially Italian.
Japanese has no native "v" sound. They can pronounce "v" (or something like it) but since "v" a foreign sound (used exclusively in loan-words) and "b" isn't it's easier to say the far more common "b" instead. (Hence, the transformation of the Norse "Verdandi" into "Belldandy" in Ah! My Goddess, and many cases of confusion between "Vulcan cannon" and "Balkan cannon.") This is why glyphs that indicate the consonant "v" only exist in the Katakana syllabary, and not in Hiragana.
The fact that "L" and "R" are the same consonant in Japanese (or, more accurately, that Japanese lacks both but has a sound that sounds to English speakers somewhere in the middle, with a bit of "d" thrown in there for good measure) is now the stuff of legend, of course. However, the consonant in question is actually not that far off from the Spanish/Italian "r". This sound is called a flap, and is much more prevalent in world languages, whereas the English 'r' is an alveolar approximate and much rarer. The Japanese 'r' is already present in the phonetic inventory of (American) English, but it's usually written as "d" or "dd". When you say the word "pudding" at normal conversational speed, without enunciating carefully, the middle consonant is the same as the Japanese 'r' (indeed, the word "pudding" is written "purin" in Japanese).
While English verbs tend to have very specific conjugation to describe time, Japanese instead tends towards very specific verb conjugation to express social standing, emotion, and opinion. The different levels of politeness associated with verbs and forms of verbs also determine whether the implied subject is the first person, second person or third person.
Trailing off at the end of a sentence and not saying something when the listener should understand it, or aposiopesis, is extremely common in (spoken) Japanese.
There are two ways of "alphabetizing" lists by phonetic readings. The modern order, gojūon-jun ("50 sounds order," though less than 50 sounds are actually used), is logical and proceeds first through the vowels (in order: [a, i, u, e, o]), then through the nine initial consonants ([k/g, s/z, t/d, n, h/f/b/p, m, y, r, w]note The sound-changing diacritics on some consonants don't change the order), also in [a, i, u, e, o] order; though not in general use before World War II, gojūon-jun is in fact the older system, having been inherited from Sanskrit via Buddhist scriptures. The pre-reform system, iroha-jun, is much weirder and orders the characters according to a poem that uses each of them once (kinda like "The quick brown fox jumps over the lazy dog"); it gets its name from the initial 3 characters I, ro, ha ("ee", "row", "hah"), which translate as "as for the color." No one uses this system for much anymore, but the term iroha is still often used much like "ABC" in English.
An example would be the Dojin created by the Genshiken crew. Its title is Iroha gokko, which could mean roughly "Playing at ABCs". However, "Iro" doesn't just mean "color". It also is used in a bunch of combinations to mean things like "sensual", or "sexy" (same character too, fun language). "Playtime" is obviously a bit naughty (well it is a dojinshi).
In Ranma ½, the sign on the wall of the Tendo Dojo reads "iroha", although it is sometimes shown in reverse in the anime.
Irohazaka, which shows up in Initial D, comes from "Iroha" and "zaka", the latter meaning "trail", from a series of distance markers along the original road which were Hiragana letters placed in the old alphabetical order. If the name was translated, it would be something like "Alphabet Road".
Just for note, it's a scenic road in the famous resort town of Nikko, Tochigi prefecture, which leads from the Tokugawa Ieyasu's mausoleum in the foothills to the lake Chuzenji and Kegon falls up in the mountains. It has 48 (which is why they are marked by Hiragana letters) extremely tight hairpin turns that are extremely challenging to drift through — and this is the reason for its inclusion into the Initial D franchise.
The Japanese don't have a concept similar to cussing or curse words like the US or UK do. While some words do literally translate to a cuss word, for example "kuso" literally means "shit", it doesn't carry the same negative social cues as it does in the US. In fact, anime aimed at children may say "kuso," "chiku shou" or "shimatta" (which tends to be translated as "dammit"). But both cultures understand that the word is used as a strong interjection. The only words that Japanese publications must bleep out are related to genitalia... Which is strange since this rule applies to hentai. Most of the time.
Japanese does, contrary to popular belief, have "official" swear words, that is, words that in their basic, non-mangled forms are inherently offensive. An example is くたばれ (kutabare), which can translate out to such charming things as "fuck off" and "drop dead". This is, as one might guess, an inherently offensive word. However, most Japanese "swear words" are indeed mostly contractions, corruptions or manglings of other words that aren't inherently offensive (unlike most Western swears), which has led to the myth that the Japanese language does not have any "real" swear words.
Japanese has a good bit of reduplication, usually used to indicate some degree of vagueness; there's even a character used to indicate repetition of the previous kanji (々). As such, you end up with words like 時々 "tokidoki" (sometimes, "time-time"), 黙々 "mokumoku" (mute, "silence-silence"), 中々 "nakanaka" (rather) or 我々 "wareware" (we "I-I").