The keyboard app uses large dictionary files for auto correction and
word suggestions. If we include all the dictionaries we have, our
builds become too large. So the keyboard is configured at build time to
include only the IMEs, layouts, and dictionaries we need for a
particular build.

Dictionaries files currently only apply for the latin IME and live in
js/imes/latin/dictionaries, along with the source files (in XML) in
Android format. There is a Makefile present to convert the source files
into Firefox OS dictionaries.

At build time we only package the IMEs and dictionaries required by the
specified keyboard layouts.

We also configure the manifest.webapp file at build time, to add entry
points for each layout.

Configuration is done with the GAIA_KEYBOARD_LAYOUTS environment
variable, and there is a default value defined in the Makefile.

All dictionaries associated to layouts included in GAIA_KEYBOARD_LAYOUTS
will be preloaded into the build. Additional layouts will not be made available
in the build unless you set these layouts in GAIA_KEYBOARD_DOWNLOADABLE_LAYOUTS
variable. Layout set in that variable will be offered as downloadable layouts
in Settings > Built-in Keyboards.

GAIA_KEYBOARD_DOWNLOADABLE_LAYOUTS is default to "noPreloadDictRequired", which
is the special keyword match all the layout that can be downloaded, layout with
no dictionary required. The other keyword is "*", which simply packages all of
the layouts.

The build happens in gaia/build_stage.
The actual configuration code is in build/layout_configurator.js.

The build tests can offer some clue on configuration and the resulting build.