People sometimes ask me how I write my name in Japanese.
Before I answer that a brief bit of background. If you're already familiar with the Japanese writing system you can skip this paragraph and the next. Japanese uses three sets of characters: katakana, hiragana, and kanji. Kanji are Chinese characters; they're not interesting when writing a non-Japanese name. Katakana and hiragana (together referred to as "kana") are syllabaries, that is, each character represents a full syllable, not just a single letter. The vowels have their own characters, the consonants do not. So while there is a kana character for "a", and there is a kana character for "ka", there is no character for "k" alone. The only exception is the "n", which does have it's own character (but, writing "na" with the character for "na" or with the characters for "n" and "a" is not entirely pronounced the same since "n" is still treated as a syllable; "n" also cannot occur at the start of a word). Katakana and hiragana use different symbols for the same sounds, the difference lies in their usage. Hiragana for Japanese words (e.g. to indicate the pronounciation of a word written with kanji), katakana for foreign words and names.
So when writing a non-Japanese name, we're interested in katakana. Because it's a syllabary, there's no direct mapping from Roman to katakana. In fact, it's often best to just ignore the Roman spelling completely and just try to approximate the pronounciation as best as possible. The biggest problem is, as indicated above, the lack of stand-alone consonants. So any consecutive consonants have to be "padded" with vowels. The most common choice for this is the "u". For instance, Amsterdam would have to be padded to become A-MU-SU-TE-RU-DA-MU (アムステルダム). By sort of swallowing the extra u's it sounds like Amsterdam again.
Unfortunately, Sven is a rather difficult name to write in Japanese, because they have no "v" sound. To approximate the "v", either a "b" is used or, more commonly, a ウ "u" with a daku-ten: ヴ. A daku-ten, which is two little marks next to a character, is normally used to turn a voiceless consonant into a voiced one, e.g. カ "ka" with a daku-ten becomes ガ "ga"). But since a "u" with daku-ten would be pronounced as "vu" not "ve", a small version of the character for "e" is added: ヴェ.
The result of this looks like this: スヴェン, which is "SU-VE-N". Close enough.
My last name also poses problems. The first is that anyone who just sees "Groot" assumes it's English and pronounces it as such. In truth, the g must be gutteral and the oo must not be as the oo in "root" but more like the oa in "road".
The Japanese also have no gutteral g, so I must settle for the g sound as in "good". Again, extra vowels must be added, so it's グ "gu". The oo sound is easier: the character ロ "ro" followed by a lengthening stroke ー gets the right pronounciation. The t becomes ト "to". Here an "o" is used as the extra vowel because the "tu" character is actually pronounced like "tsu" and not like "tu".
This then is: グロート, "GU-RO---TO".
When putting them together there's two choices. I can use western name order or Japanese name order (Japanese puts the family name first). Although western name order is the only correct one for my name, many places insist you use Japanese name order.
Completely it then becomes, in Japanese order: グロート・スヴェン.
Add さん "san" (neutral), 君 "kun" (familiar, used mainly by men), 様 "sama" (honorific) or any other common name suffix as desired. :)