Start documenting iD's mulitlingual support (re: #7713)

ACCESSIBILITY.md

@@ -38,6 +38,46 @@ elsewhere in this document.
 
 [#7132]: https://github.com/openstreetmap/iD/issues/7132
 
---
+## Multilingual Support
+
+English is the language of tags and relation roles in the OpenStreetMap database.
+It's also the source language of iD's interface, meaning English is the only language
+guaranteed to have 100% coverage. Despite this privileged position, English proficiency
+should not be a barrier to mapping.
+
+Most of iD's interface can be translated to essentially any written language via the Transifex
+platform. Some languages have region-specific variants, such as Brazilian Portuguese (`pt_BR`).
+Translators are typically volunteers. See the [translation guide](https://github.com/openstreetmap/iD/blob/develop/CONTRIBUTING.md#translating)
+for more info.
+
+|  | Localization Feature | Notes |
+|---|---|---|
+| ✅ | Detect the browser's locale | |
+| 🟩 | Right-to-left layouts | Infrequently tested. Used for languages like Hebrew and Arabic |
+
+### Translatability
+
+The following table details which interface elements can adapt to the mapper's
+language preferences. This doesn't account for whether they've actually been
+translated to one or more languages.
+
+|  | Interface Element | Notes |
+|---|---|---|
+| ✅ | Labels and descriptions | |
+| ✅ | Help docs and walkthrough | |
+| ✅ | Letter hotkeys | e.g. <kbd>S</kbd> for Straighten makes sense in English, but not every language |
+| ✅ | Preset names and search terms | |
+| 🟠 | Fields | Combo fields may show raw tag values. The Wikipedia field lists Wiki names in their native languages |
+| ❌ | Tags | OpenStreetMap tags are English-only as a limitation of the database |
+| ❌ | Relation member roles | OpenStreetMap roles are also limited to English |
+| ✅ | Imagery metadata | |
+| 🟠 | Locator overlay | This layer shows feature labels in their local languages |
+| ✅ | OSM community index | |
+| ✅ | iD validation issues | |
+| ✅ | KeepRight issues | |
+| ✅ | ImproveOSM issues | |
+| ✅ | Osmose issues | Translated strings are [provided by Osmose](https://www.transifex.com/openstreetmap-france/osmose/) itself, not iD |
+
+---
 
 _Further sections coming soon…_