Somewhere in your application a heading reads menutab__content, or a breadcrumb reads module__myapp_thing, or the administrator restrictions screen lists raw keys where names should be. The page works. Nothing is logged. It simply looks unfinished to whoever is paying you.

Invision Community renders a missing language key as the key itself. That is a sensible fallback and a poor advertisement, and there are two distinct ways to walk into it.

1. Keys you forgot to define

Every module in data/modules.json needs a module__<app>_<module> string. They appear on the administrator restrictions screen and in front-end breadcrumbs, so a missing one is customer-visible, not just staff-visible.

{
    "admin": { "settings": { ... } },
    "front": { "podcast":  { ... } }
}

/* needs BOTH */
'module__myapp_settings' => "My App",
'module__myapp_podcast'  => "My App",

Audit it in one line rather than trusting your memory:

python3 -c "
import json
m=json.load(open('applications/myapp/data/modules.json'))
lang=open('applications/myapp/dev/lang.php').read()
for area,mods in m.items():
    for mod in mods:
        k='module__myapp_%s' % mod
        print(k, 'present' if k in lang else 'MISSING')"

This was written up once here, then walked into twice more on later applications — including one that had already been published, where the restrictions screen used to decide which staff can touch what was listing internal keys. It is worth running on every app, not remembering.

2. Keys that were never yours to define

The second kind is subtler. You reference something that does not exist, and because the fallback is the key itself, it looks like a missing translation rather than a wrong value.

data/acpmenu.json puts your menu under a tab. There are exactly six, and content is not one of them:

grep -oE "menutab__[a-z]+" applications/core/dev/lang.php | sort -u
#   menutab__community
#   menutab__core
#   menutab__customization
#   menutab__members
#   menutab__stats
#   menutab__support

Use community for anything member-facing, members for member management, customization for appearance. Inventing content because it describes your app best produces a sidebar heading that reads menutab__content, and no error at all.

Related: addToStack returns a placeholder, not text

A third way to get a machine-readable string in front of a person, and the one most likely to end up in stored data rather than on a page. addToStack() returns a token for the language stack to substitute at render time. In a task, a queue, a CLI script or a feed generator, nothing renders — so the token is what gets saved:

/* wrong in a task: writes a 32-character hash as the topic title */
$topic->title = $lang->addToStack( 'my_title' );

/* right: returns the string, and throws if the key is missing */
$title = \IPS\Lang::load( \IPS\Lang::defaultLanguage() )->get( 'my_title' );

See addToStack() often returns a placeholder, not translated text for the full version. Between them these three account for most raw strings a customer ever sees.

The reliable way to catch all of it

None of these throw, none fail a lint, and none fail a test that checks a page returns 200. Look at the rendered screen. Capturing screenshots of your own AdminCP and front-end pages as a matter of routine — not only when a listing needs images — is how every one of these was actually found.

Related: A module shows as module__myapp_thing on the administrator restrictions screen, A core/Notifications extension needs a language key named after the CLASS, Raw language keys in the AdminCP menu: acpmenu.json must be an object

Related application: Discord Integration — Discord Integration names every module and menu properly, so the administrator restrictions screen reads in English rather than in internal keys.