diff --git a/docs/ARCHITECTURE.md b/docs/ARCHITECTURE.md
index cd1d174..0e168bd 100644
--- a/docs/ARCHITECTURE.md
+++ b/docs/ARCHITECTURE.md
@@ -307,7 +307,7 @@ rewriting the document.
 | 2 | Displays are bus subscribers; no display-specific logic upstream | Accepted | 2026-08-14 |
 | 3 | Listen-only sniffing + active OBD-II, single writer | Accepted | 2026-08-14 |
 | 4 | MQTT as the bus | **Proposed** | 2026-08-14 |
-| 5 | Implementation language(s) per service | **Open** | — |
+| 5 | Rust for on-Orin services, Python for off-car tooling, Dart for the display client | Accepted | 2026-09-04 |
 | 6 | Local logging first; off-board LTE telemetry deferred | Accepted | 2026-08-14 |
 | 7 | Camera/AI inference is a later, separate task | Accepted | 2026-08-14 |
 | 8 | Self-hosted offline navigation (MapLibre GL + Valhalla + PMTiles) over a cloud maps SDK | Accepted | 2026-09-03 |
diff --git a/docs/REPOS.md b/docs/REPOS.md
index 7bf9508..b4092b7 100644
--- a/docs/REPOS.md
+++ b/docs/REPOS.md
@@ -129,6 +129,20 @@ docs/impl/reports/
 docs/BOM.md               only where the repo owns hardware
 ```
 
+### Languages
+
+| Where | Language | Why it is there |
+|---|---|---|
+| `svc-*` — on the Orin | **Rust** | No GC, low resident memory on a shared 8 GB pool, and fast start for the tier-1 capture services |
+| `tool-*` — off-car | **Python** | Batch work where development speed is the only constraint that binds |
+| `app-flutter` | **Dart** | Flutter |
+
+Python floors at **3.10**, the Orin's version, for anything that might run there.
+The workstation has 3.14; do not rely on it.
+
+Rust binaries are built for `aarch64-unknown-linux-gnu`. `Cargo.lock` and
+`pubspec.lock` are committed — these are applications, not libraries.
+
 ### Writing style
 
 Documents describe **what is and what will be**. They are not changelogs — git