Enfiando co que estabades a falar e para que quede na conversa, cito:
(wikipedia EN) plain text is a term used for an ordinary "unformatted"
sequential file readable as textual material without much processing.
[...]The encoding has traditionally been either ASCII, one of its many
derivatives such as ISO/IEC 646 etc., or sometimes EBCDIC. No other
encodings are used in plain text files which neither contain any
(character-based) structural tags such as heading marks, nor any typographic
markers like bold face, italics, etc.
co cal "plain" fai referencia á ausencia de marcas de formato; que se pode
expresar como que carece de "adornos" ou é "simplemente texto".
Do Merrian Webster, 10º ed.:
PLAIN
intransitive verb
archaic : COMPLAIN
noun
1 a : an extensive area of level or rolling treeless country b : a broad
unbroken expanse
2 : something free from artifice, ornament, or extraneous matter
adjective
1 archaic : EVEN, LEVEL
2 : lacking ornament : UNDECORATED
3 : free of extraneous matter : PURE
4 : free of impediments to view : UNOBSTRUCTED
5 a (
: evident to the mind or senses : OBVIOUS (
: CLEAR b : marked by outspoken candor : free from duplicity or subtlety :
BLUNT
6 a : belonging to the great mass of humans b : lacking special distinction
or affectation : ORDINARY
7 : characterized by simplicity : not complicated
8 : lacking beauty or ugliness
adverb
: in a plain manner
adverb
: ABSOLUTELY
Así que desas tres acepcións que se recalcan: "plain text" > "texto simple",
"texto sen formato", "texto puro", "texto limpo"... o que queirades, excepto
"plano", pois de que eu saiba en galego plano non ten a acepción de simple,
sen ornamentar ou similar. Eu simplemente escollería "simple" e non teño
claro se hai que establecer unha dualidade na tradución con respecto a que
punto de vista (usuario/máquina) se faga referencia (a priori, véxoa
innecesaria).