How does Japanese handle sounds outside the 五十音図【ごじゅうおんず】? Are there ways of distinguishing sounds such as V or L in katakana renderings of foreign words? How are the missing sounds in the ワ column represented?
Heads up: Some of this is going to be a bit obscure. Wikipedia covers some of this ground; examples consisting of proper names, place names, etc. were checked via Japanese Wikipedia articles.
Due to holes in the ワ column (including the general restriction of 「ヲ」 to grammatical duties), 「ウ」can pair with other vowels to replicate /w/ combinations. 「ウ」 on its own fills the role of /wu/. Examples include:
As they are considered archaic, 「ヰ」 and 「ヱ」 are not typically used. Along with 「ヲ」, however, they will sometimes appear when a writer or graphic designer wants a certain visual impact.
All of these are in common use, and are rarely to never replaced with their counterparts in the ハ column (where 「フ」 resides) on the basis that they represent a sound not used commonly in Japanese. When Romanized, however, 「フ」on its own can be subject to some variation between F and H, for lack of a /hu/ sound in Japanese. The ハ column is relatively unique in this blend of two consonants.
Usually used in the context of Chinese names (e.g. クヮン = Kwan) or phonetic spellings of historical Japanese words.
ウ with a 濁点【だくてん】 added to it represents V, as in the following:
While these combinations exist, however, using the corresponding character from the バ column is preferable as it matches the pronunciation that will be used in practice (e.g. バルブ、ビデオ、ブ、ベイン、and ボルテックス)
This is used to get an explicit "ti" or "di" sound, as opposed to "chi" or "dji"
「ティ」is usually replaced with 「チ」for the same reason 「ヴ」usually becomes 「ブ」(e.g. バプチスト). 「ディ」 is in comparatively common use on the other hand, as the /di/ sound does not exist natively in Japanese. Using 「ヂ」 in its place is not common.
Just as the previous set provided a regular alternative to "chi" and "dji", these ones add a regular alternative to "tsu" and "dzu":
「トゥ」can be replaced with 「ツ」, and many times is (e.g. 「ツー」 “two”). Depending on the circumstances, however, it is may be left in place. This is especially the case for standardized spellings of names (e.g. 「トゥーレ」 “Touré”).
「デュ」is not usually replaced with 「ヅ」 for the same reason 「ヂ」doesn’t replace 「ディ」 above; that would require the use of the uncommon kana ヅ.
Rest from here on are almost purely academic, and most people will never encounter them in actual practice. The following add 濁点 and 半濁点 to characters that normally would not take them to allow for uncommonly-specific renderings of katakana words, as one might find in linguistic texts.
This is another (archaic) method of rendering V. It is created by adding a 濁点 to the characters in the ワ column:
As noted with the other way of representing V, ウ acts as a stand-in for /wu/.
Adding a 半濁点 to kana in the カ column produces an initial "ng" sound, like can be found in a lot of Vietnamese or Cantonese words:
In practice these words usually end up being written using the corresponding ガ行 character instead.
Adding a 濁点 to kana in the ラ column is Japanese's way of distinguishing between L and R in writing.