From a general point of view, relevant qualities for this task are: (2.2.1/601) Functionality, (2.2.2/600) Reliability, (2.2.3/603) Usability, (2.2.4/606) Efficiency.
The most important detailed features for this type of work are:
Terminology precision (2.2.1.2.3/175) - how precisely does the system translate subject-matter terminology; can it differentiate between words that translate differently in slightly different domains.
Extensibility: changeability or ease of upgrading (2.2.5.2/213), in particular dictionary update (2.2.5.2.3/-) and improveability (2.2.5.2.2/215) - can the user dynamically add new words or phrases to the lexicon; how much effort is involved.
Adaptability (2.2.6.1/221) - can the system be tuned to recognize some types of material (based on domain or genre) and handle them differently (say, more quickly).