A lot of languages have "romanization" schemes- ways to write the language using Roman or English letters. Chinese uses Pinyin, Japanese uses Hepburn, Arabic uses a collection of formal and informal ones, and so on. These tend to be somewhat systematized so there's a degree of standardization.

The Japanese specifically use Romaji which is a standardized phonetic transition from their characters/phonetics to the Latin alphabet that we know and love.

Other languages have different systems.

Some of them do guess, though. There are more languages than there are standardized systems.

In general, when translating names from a language that uses characters to one that uses the Latin alphabet (such as English), the translator will usually try to use letters that are as close as possible to the original characters. For example, if the name is written in Japanese, the translator might use "K" for "き" and "M" for "ま". However, there is no single standard for this, so the translator has some flexibility in how they choose to represent the name in English. Ultimately, it is up to the individual to decide what letters they prefer, as long as the result is still easily recognizable.

Writing the sounds of one language, into the text of another language is called transliteration. It happens naturally, when people simply write what they hear, when listening to a native speaker use words or names in the original language. There's also standardized version of it, to ensure the same spelling for the same word. In some languages it's more common to use standardized versions than in others.

When it happens naturally, before a standard version become popular, you can end up with multiple spellings and pronunciations of the same word. For example, besides the most common "Japan", the name of the country has been written as Nifon, Nihon, Nippon, and Jippon.

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