As someone who has learnt four foreign languages as an adult and worked in an international environment, a lot depends on aptitude. Only pick Chinese, Japanese, Korean or Thai if you are truly gifted and pick up languages easily. Of these Chinese will be the most useful. However be aware that this then pushes you towards a career focussed on East Asia. Does he have a real interest in Chinese culture? I knew people who spent two years learning Chinese, did a full posting and then really had no desire to return. Equally others who had pent their gap year teaching English in China and could not wait to get back.
Arabic falls into the next most difficult category along with Farsi and others. There are a lot more postings available, either with the FCDO, or commercial employers like banks and accountants. But again is he keen to spend a career based around the Middle East? Again some are, but others, especially when they have families, are not.
Russian and other Slavic languages are quicker to lean, and/or require less aptitude, but still relatively challenging. French, Spanish, German are even more straight forward and typically you would be expected to master a working level in about three months. French is used within the EU, and documents would not be translated for Civil Servants working in EU facing jobs. With translation apps this has become easier still.
In short, factor in interest in the Country/culture and general aptitude. Chinese only if you are top of the year in languages at school.
The alternative is immersion. Spend a year or two in the country and study hard whilst there and make an effort to have local friends. However be aware of new Government residency requirements:
"If a role requires national security clearance, you must meet specific continuous UK residency criteria to allow meaningful background checks: [1]
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Counter Terrorist Check (CTC): Normally 3 years of continuous UK residency.
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Security Check (SC): Normally 5 years of continuous UK residency.
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Developed Vetting (DV): Normally 10 years of continuous UK residency."
These are a complete pain, and some departments are applying them very strictly. (A PhD undertaken at a top US University ruling you out of economist jobs at the Treasury!)
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