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No (with caveats) -- Language-specific challenges are on-topic

Challenge authors are allowed to add other arbitrary restrictions (banning built-ins, banning capital letters, requiring out to be base-10 integers, etc.), and so requiring all answers to be in one language should be treated no different; it's just another method of restricting answers.

###That said ...

• The challenge should explicitly require some feature of that language to make sense. Arbitrarily restricting a challenge to a particular language should be downvoted/off-topic'd appropriately. An example would be the various regex-golf challenges, which require the answer(s) to be regex for the challenge to make sense, or the Official Dyalog APL 2016 Year Game
• If a challenge author desires a particular language answer, but is open to allowing all languages to compete, they should consider making the challenge all-inclusive with a bounty for an answer in that particular language. This should be the default.

No (with caveats) -- Language-specific challenges are on-topic

Challenge authors are allowed to add other arbitrary restrictions (banning built-ins, banning capital letters, requiring out to be base-10 integers, etc.), and so requiring all answers to be in one language should be treated no different; it's just another method of restricting answers.

That said ...

• The challenge should explicitly require some feature of that language to make sense. Arbitrarily restricting a challenge to a particular language should be downvoted/off-topic'd appropriately. An example would be the various regex-golf challenges, which require the answer(s) to be regex for the challenge to make sense, or the Official Dyalog APL 2016 Year Game
• If a challenge author desires a particular language answer, but is open to allowing all languages to compete, they should consider making the challenge all-inclusive with a bounty for an answer in that particular language. This should be the default.

No (with caveats) -- Language-specific challenges are on-topic

Challenge authors are allowed to add other arbitrary restrictions (banning built-ins, banning capital letters, requiring out to be base-10 integers, etc.), and so requiring all answers to be in one language should be treated no different; it's just another method of restricting answers.

###That said ...

• The challenge should explicitly require some feature of that language to make sense. Arbitrarily restricting a challenge to a particular language should be downvoted/off-topic'd appropriately. An example would be the various regex-golf challenges, which require the answer(s) to be regex for the challenge to make sense, or the Official Dyalog APL 2016 Year Game
• If a challenge author desires a particular language answer, but is open to allowing all languages to compete, they should consider making the challenge all-inclusive with a bounty for an answer in that particular language. This should be the default.

No (with caveats) -- Language-specific challenges are on-topic

Challenge authors are allowed to add other arbitrary restrictions (banning built-ins, banning capital letters, requiring out to be base-10 integers, etc.), and so requiring all answers to be in one language should be treated no different; it's just another method of restricting answers.

###That said ...

• The challenge should explicitly require some feature of that language to make sense. Arbitrarily restricting a challenge to a particular language should be downvoted/off-topic'd appropriately. An example would be the various regex-golf challenges, which require the answer(s) to be regex for the challenge to make sense, or the Official Dyalog APL 2016 Year Game
• If a challenge author desires a particular language answer, but is open to allowing all languages to compete, they should consider making the challenge all-inclusive with a bounty for an answer in that particular language. This should be the default.