i18n phases 1+2: BAL translation layer and translatable texts

Phase 1: new bal/i18n.py, BAL's own gettext layer (domain "bal", catalogs
read with plugin.read_file()). _() asks Electrum's catalog first, then
BAL's, then returns the English source. The Qt plugin loads the catalog of
Electrum's GUI language at start-up; the CLI stays English.

Phase 2: every user-visible GUI text is now a whole, extractable sentence
(Ruff INT rules enabled). Class-level texts are marked with N_() and
translated when shown. Stored data stays language-neutral: the status
history is written in English and translated for display, the calendar
defaults follow the GUI language, and the history label and wallet labels
are never translated because BAL uses them to recognise its transactions.

No visible change apart from the double colon fixed in the will detail.
See CHANGELOG entries 58 and 59 and PLAN_I18N.md.

bal/core/plugin_base.py

@@ -32,6 +32,8 @@ from electrum.plugin import BasePlugin
 from electrum.transaction import tx_from_any
 from electrum.util import classproperty
 
+from ..i18n import N_, _
+
 _logger = get_logger(__name__)
 
 
@@ -134,12 +136,20 @@ class BalConfig:
 
     Wraps ``config.get`` / ``config.set_key`` and supplies a default value
     when the key is missing.
+
+    ``translatable=True`` is for a default *text* (marked with ``N_()``) that
+    should follow the GUI language. The English default is what gets stored,
+    and :meth:`get` translates it when it is read, so the stored value never
+    depends on the GUI language. A text the user wrote is returned unchanged.
+    Only for texts that are shown: a text that is also used to recognise
+    stored data (e.g. ``HISTORY_LABEL``) must stay non-translatable.
     """
 
-    def __init__(self, config, name, default):
+    def __init__(self, config, name, default, translatable=False):
         self.config = config
         self.name = name
         self.default = default
+        self.translatable = translatable
 
     def get(self, default=None):
         """Return the stored value, falling back to ``default`` then ``self.default``."""
@@ -149,10 +159,20 @@ class BalConfig:
                 v = default
             else:
                 v = self.default
+        if self.translatable and v == self.default:
+            return _(v)
         return v
 
+    def localized_default(self):
+        """Return the default value, translated if the setting is translatable."""
+        return _(self.default) if self.translatable else self.default
+
     def set(self, value, save=True):
         """Persist ``value`` for this key."""
+        if self.translatable and value == self.localized_default():
+            # The translated default is stored as its English original, so
+            # it keeps following the GUI language (see get()).
+            value = self.default
         self.config.set_key(self.name, value, save=save)
 
 
@@ -243,6 +263,10 @@ class BalPlugin(BasePlugin):
         # the wallet's local history. May contain the "{willexecutor}" token,
         # which is replaced with the will-executor URL of each will item at
         # save time.
+        # Deliberately NOT translatable: Util._label_matches_history() uses
+        # this text to recognise BAL's own local transactions (stale-history
+        # cleanup, spendable UTXOs). A label that changed with the GUI
+        # language would no longer match the transactions saved before.
         self.HISTORY_LABEL = BalConfig(
             config,
             "bal_history_label",
@@ -328,13 +352,20 @@ class BalPlugin(BasePlugin):
         self.WELIST_SERVER = BalConfig(
             config, "bal_welist_server", "https://welist.bitcoin-after.life/"
         )
+        # Calendar (.ics) texts: only shown, never used to recognise data, so
+        # their defaults follow the GUI language (translatable=True). The
+        # $tokens must survive translation.
         self.EVENT_DESCRIPTION = BalConfig(
             config,
             "bal_event_description",
-            "BAL will execution of $wallet_name\r\n heirs list:  \r\n$heirs_complete",
+            N_("BAL will execution of $wallet_name\r\n heirs list:  \r\n$heirs_complete"),
+            translatable=True,
         )
         self.EVENT_SUMMARY = BalConfig(
-            config, "bal_event_summary", "BAL -Will execution of $wallet_name"
+            config,
+            "bal_event_summary",
+            N_("BAL -Will execution of $wallet_name"),
+            translatable=True,
         )
 
         # Default will-executor servers, keyed by network.  These addresses are