Finish change from “Bengali” to “Bangla”

“Bengali” still remained in some comments in the bn_BD locale file,
in iso-639.def and in a test input file. Change it there as well.
“Bangla” is now used as the English name for this language in CLDR.

	[BZ #14925]
	* libio/tst-widetext.input: Change “Bengali” to “Bangla”.
	* locale/iso-639.def: Change “Bengali” to “Bangla”.
	* localedata/locales/bn_BD: “Bengali” was still used in some
	comments. Change it to “Bangla”.