How do I add a language to the pick list for formatting characters?

I want my OpenType font to use the right style for the language. For example, I want to be able to pick Tai Lue, ISO 639-3 code khb, OpenType tag XBD, Microsoft language code apparently given by this line in lang.h:
#define LANGUAGE_LU_CHINA 0x0490

(I have a longer list, but first I need to understand if simply asking for them will provide the ability to use them.)

This question applies to LibreOfice, UI language English, document (or cell or paragraph) language - Tai Lue, component: Writer, Calc (probably all)

Operating system: Ubuntu 12.04 (English), but Windows (Thai) is also desirable.

The relevant script is Tai Tham (ISO code Lana, OpenType tag lana). Selecting the language gets the right style for Lao in the Tai Tham script.

One can’t do this for complex or CJK scripts. It’s a matter of design - for now at least. For Western scripts, one can just type the language tag in.

For Tai Lue at least, one can create a macro to change the selection. See
How do I set the locale of the current selection for details. Thanks to Pierre-Yves Samyn for the missing piece in the work around.