You need to speak with your potential supporters, their purpose is to confirm that your application is "true and fair". It may be possible to explain to a supporter how the evidence confirms what is written in English within the application form
The IET will not ask you to send a large portfolio, although it may prove valuable if questions arise. Some people like to bring something like this along to an interview and a well constructed portfolio can help to support a presentation. Diagrams charts calculations etc shouldn't need much translation. The IET is required by Engineering Council rules to assess in English (or Welsh by special request).
Seek the support of a Professional Registration Advisor (PRA) so that your application is presented to your best advantage.
Roy Bowdler IEng FIET FCIPD
IET Registration & Standards