Do you need to translate a parent consent for travelling (flying) abroad?

Yes, you need a translated version of the letter as well, as UK immigration require all the foreign language document should be translated into English ,

here is the reference,

https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/549692/Visitor_Supporting_Documents_Guide_-_English_version.pdf

If you submit a document that is not in English or Welsh, it must be accompanied by a full translation that can be independently verified by the Home Office. Each translated document must contain:

• confirmation from the translator that it is an accurate translation of the original document
• the date of the translation
• the translator's full name and signature
• the translator's contact details

Under 18s... If travelling unaccompanied or with someone other than your parent(s) you should provide:

• a signed letter from your parent(s) confirming details of anyone accompanying you, and details of care and accommodation arranged in the UK
• a copy of your parent(s) or legal guardian’s biographical page of their passport, which includes their signature and passport number if your parent(s) do not have a passport then you must provide another official document which bears their signature