Document the nine languages, and what Beta means on them

The translations shipped today and the docs still said "no languages but
English". They also need to say the harder thing, which is that eight of
the nine have never been read by anybody who speaks them.

- README gains the language list, with the Beta caveat in the same line
  rather than a footnote.
- FEATURES.md gains an Interface language section: the list, why it is a
  separate setting from the date locale, and the two design properties
  that follow — a missing entry renders English, and plurals are asked of
  Intl.PluralRules rather than assumed, which is why Russian carries
  three forms and Japanese one.
- ROADMAP.md no longer lists translations as "not yet". What replaces it
  is the half that is genuinely not done: a translation anybody has
  checked. RTL is split out as its own entry, because holding Arabic,
  Hebrew and Persian back is a layout decision and not a queue position.
- KNOWN-ISSUES.md gains two entries. The unread catalogues, which is the
  one thing on that page that cannot be closed by testing. And the
  coverage number that read 100% while two hundred strings rendered
  English in every language — recorded as a general lesson rather than an
  i18n one, since a coverage number measures what it can see and the rest
  is exactly what nobody is checking.

FEATURES.md

@@ -594,7 +594,7 @@ not reach another that already has ihasmail open until it signs in again.
 | Section | Holds |
 | --- | --- |
 | **General** | Reading pane, mark-as-read delay, auto-advance, remote images, conversation view, snippets, avatars, confirm-before-delete; compose format, undo-send window, quoting, signature placement, attachment reminder, read-receipt policy, spell check; time zone, week start, language & region, date format, time format; `mailto:` handler; export / import / reset |
-| **Appearance** | Theme, accent colour, density, font size, sidebar, swipe actions |
+| **Appearance** | Theme, accent colour, density, font size, sidebar, swipe actions, interface language |
 | **Identities & signatures** | Addresses, names, Reply-To, HTML signatures, the default, and which to hide from the picker |
 | **Filters & rules** | The visual builder and raw Sieve editor |
 | **Out of office** | Vacation response |
@@ -629,6 +629,47 @@ Settings **export** to a JSON file and **import** back, and reset to defaults.
   not round-trip against a Gregorian grid. Non-Gregorian calendars are not
   implemented.
 
+## Interface language
+
+Nine languages, chosen in **Appearance → Language**, and separate from the
+date-and-time locale above. Wanting German dates on an English interface is a
+real preference and so is the reverse, which is why they are two settings and
+not one.
+
+| | |
+| --- | --- |
+| English | the source language, and what every other catalogue falls back to |
+| Deutsch · Español · Français · Nederlands · Português (Brasil) | Beta |
+| Русский · Українська · 简体中文 · 日本語 | Beta |
+
+**Every language but English is marked Beta, and the label is not modesty.**
+The catalogues were produced by AI against standard dictionaries and have not
+been read by anybody who speaks the language. That is stated in Settings, next
+to a link for reporting anything that reads wrongly, because the alternative —
+shipping them quietly — would ask people to trust text nobody has checked. A
+language loses the Beta mark when a speaker has read it and said so, which is a
+deliberate act by a person and not something a percentage earns.
+
+Two things follow from the design rather than the translation:
+
+- **A missing entry renders its English source.** So deleting a bad line is a
+  valid fix, not a regression, and a catalogue is never in a half-broken state.
+- **Plurals are asked for, never assumed.** `Intl.PluralRules` decides the form,
+  so Russian and Ukrainian get their three (1 письмо, 2–4 письма, 5+ писем) and
+  Japanese and Chinese get the one they actually have — with counters doing the
+  work a plural would: 通 for messages, 件 for conversations.
+
+The interface language also feeds the *automatic* date locale, so choosing
+日本語 gives Japanese month and weekday names without setting the region too.
+`<html lang>` follows it, which is what stops a browser offering to translate a
+page that is already in the reader's language — and accepting that offer is
+what rewrites the DOM underneath React.
+
+Only languages with a catalogue shipped appear in the picker. A language
+offered without strings behind it would leave the page claiming to be in a
+language it is not, which is worse than not offering it: it stops a browser
+offering to translate a page the reader cannot read.
+
 ## Themes
 
 Follow the system, light, dark, or **ihasmail** — the palette this project's
@@ -917,8 +958,9 @@ tested.
 
 The full list with reasons is [ROADMAP.md](ROADMAP.md). In short: no snooze
 (nothing in JMAP or Stalwart supports it, and ihasmail holds no password to act
-on a mailbox while you are away), no languages but English, no two-factor
-sign-in without an app password, no sharing of mail folders (the server stores
-the share and never delivers it), no public links (JMAP shares with accounts on
-the same server, and ihasmail has no storage of its own to mint a link from),
-and no per-occurrence *this and future* edits (the server refuses them).
+on a mailbox while you are away), no language yet checked by a native speaker,
+no two-factor sign-in without an app password, no sharing of mail folders (the
+server stores the share and never delivers it), no public links (JMAP shares
+with accounts on the same server, and ihasmail has no storage of its own to
+mint a link from), and no per-occurrence *this and future* edits (the server
+refuses them).