add option to override locale messages per-group, update readme

README.md

@@ -55,43 +55,74 @@ A typical setup could look like this:
 │   │   ├── control.json
 │   │   └── state.json
 │   └── chatterbox@botsin.space
-│       ├── config.json
-│       ├── control.json
-│       └── state.json
+│       ├── config.json   ... fixed config edited manually 
+│       ├── messages.json ... custom locale overrides (optional)
+│       ├── control.json  ... mutable config updated by the group service
+│       └── state.json    ... group state data
+├── locales
+│   ├── ... custom locale files, same format like en.json
+│   └── ru.json
 └── groups.json
 ```
 
+#### Locales
+
+English locale ("en") is bundled in the binary. Additional locales can be placed in the `locales` folder.
+If an entry is missing, the English version will be used.
+
+The locale file looks like this (excerpt):
+
+```json
+{
+  "group_announcement": "**📢Group announcement**\n{message}",
+  "ping_response": "pong, this is fedigroups service v{version}"
+}
+```
+
+- Words in curly braces (`{}`) are substitution tokens. These must be preserved in all translations.
+- Pay attention to line endings and blank lines (`\n`). Some messages from the locale file are combined to form the
+  final post, leaving out newlines can result in a mangled output.
+
+The locale to use is chosen in each group's `config.json`, "en" by default (if not specified).
+
+Group-specific overrides are also possible: create a file `messages.json` in the group folder
+and define messages you wish to change, e.g. the greeting or announcement templates.
+
 #### Common config
 
 There is one shared config file: `groups.json`
 
-- If the file does not exist, default settings are used. This is usually good enough.
-- This file applies to all groups
-- Prior to 0.3, the groups were also configured here. 
-- Running 0.3+ with the old file will produce an error, you need to update the config before continuing - move groups to subfolders
+- If the file does not exist, default settings are used. This is usually sufficient.
+- This file applies to all groups and serves as the default config.
 
 ```
 {
+    // name of the directory with groups
+    "groups_dir": "groups",
+    // name of the directory with locales
+    "locales_dir": "locales",
+    // Show warning if locales are missing keys
+    "validate_locales": true,
     // Max number of missed notifs to process after connect
-    max_catchup_notifs: 30,
+    "max_catchup_notifs": 30,
     // Max number of missed statuses to process after connect
-    max_catchup_statuses: 50,
+    "max_catchup_statuses": 50,
     // Delay between fetched pages when catching up
-    delay_fetch_page_s: 0.25,
+    "delay_fetch_page_s": 0.25,
     // Delay after sending a status, making a follow or some other action.
     // Set if there are Throttled errors and you need to slow the service down.
-    delay_after_post_s: 0.0,
+    "delay_after_post_s": 0.0,
     // Delay before trying to re-connect after the server closed the socket
-    delay_reopen_closed_s: 0.5,
+    "delay_reopen_closed_s": 0.5,
     // Delay before trying to re-connect after an error
-    delay_reopen_error_s: 5.0,
+    "delay_reopen_error_s": 5.0,
     // Timeout for a notification/timeline socket to be considered alive.
     // If nothing arrives in this interval, reopen it. Some servers have a buggy socket
     // implementation where it stays open but no longer works.
-    socket_alive_timeout_s: 30.0,
+    "socket_alive_timeout_s": 30.0,
     // Time after which a socket is always closed, even if seemingly alive.
     // This is a work-around for servers that stop sending notifs after a while.
-    socket_retire_time_s: 120.0,
+    "socket_retire_time_s": 120.0
 }
 ```
 
@@ -111,6 +142,7 @@ Only the `config.json` file with credentials is required; the others will be cre
 - `state.json` - frequently changing state data. The last-seen status/notification timestamps are kept here.
   State is split from Control to limit the write frequency of the control file. Timestamps can be updated multiple times
   per minute.
+- `messages.json` - optional per-group locale overrides
 
 **Do not edit the control and state files while the group service is running, it may overwrite your changes!**
 
@@ -127,9 +159,11 @@ The file formats are quite self-explanatory (again, remove comments before copyi
 {
   // Enable or disable the group service
   "enabled": true,
+  // Group locale (optional, defaults to "en")
+  "locale": "en",
   // Group account name
   "acct": "group@myserver.xyz",
-  // Saved mastodon API credentials
+  // Saved mastodon API credentials, this is created when authenticating the group.
   "appdata": {
     "base": "https://myserver.xyz",
     "client_id": "...",