The stringlist directives:
  stringlist on
  stringlist off
  stringlist
control localizability of string literals (step 1 below). The default state is off.

A string literal will be localized if:
[1] it is encountered while stringlist is on
[2] a Localizable.strings file is present in the appropriate *.lproj folder of the app's internal folder hierarchy
[3] the file contains a key that matches the original string literal
[4] the value corresponding to the key has been edited to be localized.
otherwise the original unlocalized string is used.

A localizable string takes up more memory than a non-localizable one, and access to it is much slower. Recommended practice is to place each string (or group of strings) that you want localizable between 'stringlist on' and 'stringlist' directives:

stringlist on         // turn localizability on
print "Hello, world!" // localizable
stringlist            // revert to previous state
plusSign$ = "+"       // not localizable

FBtoC's menu command 'File > Generate Localizable.strings' creates a file in the built_temp folder, with an entry for each localizable string. An entry consists of a comment (file: line) to assist the translator, followed by "key = "value";

/* HelloWorld.bas: 4 */
"Hello, world!" = "Hello, world!";

After editing the value for French.lproj:

"Hello, world!" = "Bonjour, le monde!";

See Apple documentation: Internationalization Programming Topics