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In many cases, translators specialize in certain types of subject matter. Some may almost exclusively handle mechanical patents, others medical, others chemical, and so on. Some prefer not to translate patents at all, and concentrate on advertising copy, business documents, financial statements, instruction manuals, or any number of other fields.
A client who researches, interviews, and hires an independent or in-house translator has done everything possible to ensure that the translator has skills commensurate with requirements.
Translation agencies may make an effort to send patent documents to translators who specialize in that field, and so on, but may not always do so, especially if time constraints or translator availability preclude this.