Japanese names written using kanji—logograms representing meaning—can ultimately be pronounced in any way chosen by the creator of that name. Inferring a set of kanji for a name from its English spelling is a rough guess at best and often incorrect. Likewise, when a name is provided in kanji, online dictionaries can offer some possibilities on how it CAN be read. But the only way of knowing exactly how to read the name of a particular individual is to either ask someone who already has that knowledge, or to consult a reliable document that refers to that individual and spells out how it is read.
This is related to how parents name their child in Japanese. A parent can READ a set of kanji they’ve selected for their child however they want. There is no law governing how a set of kanji must be read.
There are limits, however, on which kanji can be used for people’s names. Article 50 of the Family Register Act stipulates that the scope of such kanji be defined by the Ministry of Justice, which provides two groups of kanji for this purpose: joyo-kanji (常用漢字) and jinmeiyo-kanji (人名用漢字).
As long they are in either of these groups, parents are free to choose how the kanji are read, although in rare cases, the officer receiving the notification of birth may reject it if it is too long, or is otherwise deemed inappropriate, as was in the case of the widely-reported Akuma-chan incident in 1993, where Akuma (悪魔) can mean devil, or demon.
The above has been about first names. Although surnames can be less unpredictable, there is still a diverse array of possibilities to choose from. Here’s a couple of surnames and possible ways of reading them:
金城: Kanagushiku, Kanagusu, Kanaki, Kanashiro, Kaneki, Kaneshiro, Kinjo, etc.
神谷：Jintani, Jinya, Kamidani, Kamitani, Kamiya, Kandani, Koya, Mitani, etc.
The responsible way to deal with Japanese names, therefore, is for you to provide this information to the translator, and for the translator to ask you, look it up, and if still unsure, flag it for you.
The method to be employed depends on the document. If it is a certificate, an agreement or a contract, statements and affidavits, minutes, internal documents and so on that include the names of people likely to be known to you, asking you may be the natural course of action.
If it is an article, guidelines, specifications and other material perhaps with a bibliography, citations or a list of references, your input can still be of great help, such as when these individuals are known to you. This will complement the translator’s efforts to search the internet for materials authored by these people, for example. Names of politicians, judges, senior civil servants and other public figures are also relatively accessible online.