No! An ESL teacher has to know about it, it forms part of what is called "language awareness". The ESL teacher has to know, not the student, in order to adjust his teaching strategy to the needs of the students.

You made the same mistake when writing in a simple Spanish sentence MI as ME. Spelling and sounding of /i:/ are consistent in Spanish and in the writing systems of many other languages, but not so in English. The question of such topics is whether it is relevant for ESL teaching or not. And it is. Any PGCE or C/DELTA teacher training does not cover any of these topics; it normally should form part of your previous studies for the BA degree in English at uni. PGCE and C/DELTA should only tell you how to deal with these matters in ESL teaching. If you choose to ignore the relevance, then you choose to ignore the needs of your non-native ESL students.

It's interesting, but how the language became what it is today is not required knowledge to teach it, especially not if you are a fully trained English teacher. It's like saying a heart surgeon must know every historical advance in anatomy in order to perform surgery on a patient today.

Nice u-turn on the PGCE, something you said previously should be a requirement (in addition to relevant degree) for any teacher!

Regarding the Spanish, as said before it was a private joke between myself and my ex-colleague based on a Ricky Gervais comedy sketch about the English abroad. The whole post was private jokes about our time working together! But do carry on grasping at straws.