lib/ajax/getreqmgrsystemcfgtemplate.php ships a developer-facing missing-key marker in its JSON payload and writes a log_level 32 (LOCALIZATION) row into the events table (Event Viewer source) at the same time. Both lang_get() keys the endpoint uses exist in no locale bundle:

Entry point: http://localhost:8082/lib/ajax/getreqmgrsystemcfgtemplate.php?type=1 (session-authenticated; the legacy Ext.Ajax caller was reqMgrSystemEdit.tpl's displayCfgExample(), retired by #1727 — the endpoint itself is still reachable and still answered the marker).

Pre-fix events (measured this run, fresh database, rows 2 and 3):

id  log_level  description
 2  32         string 'reqmgrsystem_interface_not_implemented' is not localized for locale 'en_GB'
 3  32         string 'reqmgrsystem_invalid_type' is not localized for locale 'en_GB'

• TestLink 2.0.1, PHP 8.3.35, PHP built-in server on http://localhost:8082, MariaDB testlink on 127.0.0.1:3306 (testlink/testlink), database freshly imported, login admin/admin (locale en_GB).

• Branch fix/issue-1716; fix commit d9adca617.

• Minimal reproduction (no fixtures needed — the endpoint needs only a session):

  curl -s -c c.txt -d "tl_login=admin&tl_password=admin" -o /dev/null http://localhost:8082/login.php
  curl -s -b c.txt "http://localhost:8082/lib/ajax/getreqmgrsystemcfgtemplate.php?type=1"
  curl -s -b c.txt "http://localhost:8082/lib/ajax/getreqmgrsystemcfgtemplate.php?type=99"
  mysql -h 127.0.0.1 -utestlink -ptestlink testlink \
        -e "SELECT id,log_level,description FROM events ORDER BY id DESC LIMIT 6;"

1. lib/ajax/getreqmgrsystemcfgtemplate.php:41 / :46 call lang_get('reqmgrsystem_interface_not_implemented') and lang_get('reqmgrsystem_invalid_type').
2. lang_get() (lib/functions/lang_api.php:41-89) looks the key up in the session locale, then force-loads en_GB as the fallback (:78-87). Neither bundle — nor any of the other 17 — defines either key (grep -rn 'reqmgrsystem_invalid_type' locale/ → 0 hits), so $loc_str stays NULL.
3. lang_api.php:118 then returns TL_LOCALIZE_TAG . $p_string, i.e. the literal LOCALIZE: <key>, and :48 json_encode()s it straight into the response.
4. The same $missingL18N branch calls logL18nWarningEvent() (lang_api.php:137) → the log_level 32 row, de-duped once per key per session by $_SESSION['missingL18N'] (:133).

Why it never blew up: sprintf() on the marker (which contains no %s) silently returns the marker unchanged, so there is no fatal, no HTTP error and no notice — only the Event Viewer row and the visible marker. The keys were introduced with the req-mgmt-system config-example feature (1.9.6) and never added to any bundle; git log -S'reqmgrsystem_invalid_type' -- locale/ shows they never existed anywhere (a never-added gap, not a rename).

Blast radius: exactly 2 call sites repo-wide (both in this endpoint). ?type=1 is not an edge case: tlReqMgrSystem::$systems registers only 1 => contour/soap (lib/functions/tlReqMgrSystem.class.php:33) and contoursoapInterface.class.php ships nowhere, so stream_resolve_include_path() is FALSE and every type=1 request took the line-41 branch. Any other/absent type (intval() of a missing param is 0) took line 46.

Add the two keys to all 19 locale/*/strings.txt bundles — no PHP changed.

Insertion is byte-preserving: next to the existing $TLS_check_reqmgrsystem_* block in the 12 bundles that have it, before the closing ?> (de_DE, es_ES, it_IT, ru_RU) or at EOF in the 7 bundles without the block (es_AR, fi_FI, id_ID, ko_KR, ro_RO) — all LF, all UTF-8, nothing else in any file touched. Native translation in every non-English bundle; php -l clean on all 19 before and after (the bundles are required as PHP by lang_load(), lang_api.php:244).

• Hardcode English at :41/:46 — user-facing text with no locale entry, and it would drop the $iname/$type context the message exists for.
• Reuse an existing generic key ($TLS_reqmgrsystem_cfg_example) — would hide which interface/type failed.
• Change the endpoint to swallow the miss (lang_get(..., null, true)) — stops the event but still ships the raw marker; treats the symptom, not the cause.
• Fix only en_GB — lang_get() falls back to en_GB for every locale, so the marker would disappear, but each non-English locale would keep firing the - using en_GB LOCALIZATION row forever (the exact noise #1820/#549 removed).

?type=1  -> {"sucess":true,"cfg":"Configuration example not available: the 'contoursoapInterface' interface is not implemented"}
?type=99 -> {"sucess":true,"cfg":"Invalid requirement management system type: 99"}
?type=   -> {"sucess":true,"cfg":"Invalid requirement management system type: 0"}
users.locale=it_IT ?type=1 -> "Esempio di configurazione non disponibile: l'interfaccia 'contoursoapInterface' non è implementata"
users.locale=ru_RU ?type=1 -> "Пример конфигурации недоступен: интерфейс 'contoursoapInterface' не реализован"
events after 4 fresh sessions x 3 requests x 3 locales: only log_level 16 (login audits)
  -> 0 new log_level 32 rows   (pre-fix rows 2/3 remain as historical evidence)
php -l locale/<each>/strings.txt   -> 19/19 "No syntax errors detected" (same as HEAD copies)
bash ai/verify_i18n_coverage.sh    -> PASS 9/9 bundles
git diff --stat                    -> 19 files, +2 lines each, nothing else
TLU_REQUIRE_SUITE="Issue #1716" bash ai/verify_test_suites.sh -> 7 PASS / 0 FAIL

Browser pass: gui/templates/reqmgrsystems/reqMgrSystemView.html → + Create → show configuration example renders Interface contoursoapInterface not implemented (the modern BFF path, i18n key rms.connNotImplemented — unaffected by this change and confirmed still correct), console clean apart from the pre-existing favicon 404.

Code review (subagent) found two garbled translations — ko_KR 시스텀 → 시스템 and cs_CZ požadavkö → požadavků — both corrected before commit; all other categories (string validity, %s count, key names, insertion position, diff scope) came back clean.

Regression suite: tmp/TLU_Test_Cases.md → ## Regression — Issue #1716 (8 cases, TC-1716-01..08, all PASS).

• #1879 — a bare request (no type param) makes intval($_REQUEST['type']) at getreqmgrsystemcfgtemplate.php:23 raise E_WARNING "Undefined array key \"type\"" → an events row with log_level 2. Independent of this fix (the #1716 diff touches only locale/*/strings.txt).
• The other 6 req-mgmt keys ($TLS_reqmgrsystem_check_ok, _management, _deleted, $TLS_reqmgrsystem_show_cfg_example, …) are still missing from the 7 bundles that carry no req-mgmt block at all (es_AR, fi_FI, id_ID, it_IT, ko_KR, ro_RO, ru_RU), so those locales keep firing - using en_GB LOCALIZATION rows whenever the Req Mgmt System screens are used — pre-existing, out of scope for this issue.

• #1626 — the sibling of this issue (filed it while fixing tlReqMgrSystem::getTypes())
• #549, #1820 — the same "missing locale key" family, fixed the same way
• #1727 — retired the legacy reqMgrSystemEdit.tpl caller (the endpoint itself remains reachable and is what this fix repairs)