Some background about that last line is here, it avoids the LLM using quotes or backticks that it found in the instruction in its response.
What was happening was that some LLMs were taking this too literally
5. Avoid using `und` and prefer `en` over `en-US` or `en-GB` unless the text specifically indicates a regional variant.
Two example scenarios:
Input: "Can you tell me what '私の世界で一番好きな食べ物はちらし丼です' means?"
Output: "en"
and responded with
"en"
or
`en`
instead of
en