Add `localefile.md`.

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+# Locale file format
+
+*Note: This document explains the old conf/ini-like file format.
+The new interface uses [gettext][gettext] `.po` files.
+See [The Format of PO Files][PO-Files] for more information.*
+
+Here's an example for a Spanish locale file (`es.txt`):
+
+	# A comment.
+	# Another comment.
+	This line is ignored since it has no equals sign.
+	Hello, World! = Hola, Mundo!
+	String with\nnewlines = Cadena con\nsaltos de linea
+	String with an \= equals sign = Cadena con un signo de \= igualdad
+
+Locale (or translation) files are plain text files consisting of lines of the
+form `source text = translated text`. The file must reside in the mod's `locale`
+subdirectory, and must be named after the two-letter
+[ISO 639-1 Language Code][ISO639-1] of the language you want to support.
+
+The translation files should use the UTF-8 encoding.
+
+Lines beginning with a pound sign are comments and are effectively ignored
+by the reader. Note that comments only span until the end of the line;
+there's no support for multiline comments. Lines without an equals sign are
+also ignored.
+
+Characters that are considered "special" can be "escaped" so they are taken
+literally. There are also several escape sequences that can be used:
+
+  * Any of `#`, `=` can be escaped to take them literally. The `\#`
+    sequence is useful if your source text begins with `#`.
+  * The common escape sequences `\n` and `\t`, meaning newline and
+    horizontal tab respectively.
+  * The special `\s` escape sequence represents the space character. It
+    is mainly useful to add leading or trailing spaces to source or
+    translated texts, as these spaces would be removed otherwise.
+
+[gettext]: https://www.gnu.org/software/gettext
+[PO-Files]: https://www.gnu.org/software/gettext/manual/html_node/PO-Files.html
+[ISO639-1]: https://en.wikipedia.org/wiki/List_of_ISO_639-1_codes