If I'm right, the white on his flag is meant to be transparent as to make it a non-rectangular flag.
It's meant to be a swallowtail flag. The vertical white bit is part of the flag, but not the white triangular section.
As a consequence of this, Mitravarta actually has an official "national mantra": Om Mitrave Namah, the mantra for meditation on and invocation of Mitra. There is a second national mantra, which is the famous Gayatri Mantra, which when simplified, is a prayer to the sun god Savitr.
One question, do you think the mods would allow a Hindu swastika on my flag? I see Athara Magarat has one (reversed one, but a swastika regardless) on theirs.
The mods will pester you initially (and some idiots on the forums) but so long as you provide a reason that distinguishes yours from the nazi version and display it for people to see you'll be fine to keep it.
Just to check: is it allowed if an original language with English translation is given?
Context would be e.g. quotations of existing literary texts in non-English languages; giving the original would allow readers to either read that (if they know the language) or to look for alternative translations to be sure of their meaning.
I realise that such situations may not happen that often here, but just in case :)