Different rates by language

Why do translators often charge different per-word rates depending on the language pair?

I can’t speak for everyone, but it’s usually not because a translator “knows one language better than another”. Supply and demand can play a role, but it’s not the main driver either.

In principle, an hourly rate could be the same. However, there are three solid reasons why per-word rates legitimately vary by language:

Word count needed to express the same idea. Different languages need different amounts of text to say the same thing. As a rule of thumb, the same content is noticeably shorter in English than in Romance languages.
Grammar distance. Although the translator always writes in the target language, experience shows that the mental translation process is faster when the source and target languages are structurally similar.

For instance, Spanish and Portuguese are nearly parallel for many everyday sentences. French is close too, though tense usage may shift (e.g., present perfect vs. simple past in informal contexts). German often requires a different word order, and Chinese follows very different grammatical patterns. The closer the systems, the quicker the mental mapping.

Terminology. The difficulty of specialised terms varies widely by language. As with grammar, similarity helps. A Spanish speaker with no Italian might still correctly infer that spettrometro di massa isotopica means “isotopic mass spectrometer”. In other languages, the leap is not so obvious. Another key factor is the availability of resources (dictionaries, textbooks, websites) for a given language and field. On that front, English tends to be the most advantageous in most domains because there are simply more reference materials.