$LANGUAGE en
$MESSAGE Welcome
"Welcome"            # en — the $LANGUAGE
[tr]
"Hoş geldin"
[de_AT]
"Servus"

Blanks are allowed inside the brackets. A second word — a charset, [de ISO-8859-1] — still parses but is deprecated (a warning).

app.msg:2:4: error: expected a charset name or ']', found end of line

Semantics — how the generated code picks a language: Reference › Texts & locale selection.