When viewing the ACP in a non-default, user selected language, an Extension will NOT fall back to the board's default lang or English. Instead, the raw lang vars are displayed. This only appears to happen with extensions that are relying on phpBB's auto-inclusion of info_acp_lang files for the ACP (loaded by the add_mod_info function).
Language behavior is as expected for extensions in the ACP only if they are using the core.user_setup event or the add_lang_ext() methods.
It also appears that if the extension is using an ACP module, the names of the modules and its modes display as raw lang vars too, and they only work as expected when using the core.user_setup event to load the info_acp_lang file.