A service translation is a translation done out of the translator's first language.
Normally, a translator will know two (or more) langages, but will be most proficient in their mother tongue. Because of this, most translations will happen from a second (or third or fourth) language, and into the first language or mother tongue.
A service translation is the opposite: A translation from one language into a language in which one is not as proficient. In general, this practice is discouraged, as it is unlikely to be as good as if the translator would be translating into the language with which they are most comfortable.
While discouraged - and apparently even generally frowned upon in translation communities - service translations is a regular practice for many working translators, for example if trusted customers need translations done in the "opposite direction" on a short notice, and are unable to find someone who is able to do the job natively.
Nobody seems to know why a service translation is called such, but I imagined the name came from the fact that these translations are ones you would do as a favour ("service") to your customers, rather than it being the bulk of work. If anyone has a better explanation, I'd be happy to accept it.