In some cases, especially for very unusual language combinations (such as from Latvian to Chinese) it may be necessary to use a language other than the original language as the source language. In other words, you use a translation (commonly to English) as the source text to translate to the target language.
This method does, however, entail a certain risk of shifts in meaning and in the worst case, erroneous translations. In rough terms, the effect can be compared with the whispering game (Chinese whispers) where the original message is distorted the further from the originator it gets.