Improve Fluent translation cache

It was caching the `FluentLocalization` object, but that caused issues
in multithreaded environments (like the development server) due to
simultaneous attempts at iteration. This change caches loading the
Fluent resources which is the expensive work of parsing the files
and bundling the results into lists of resources.

Fix #8705

lib/l10n_utils/fluent.py

@@ -100,20 +100,8 @@ class FluentResourceLoader:
     @staticmethod
     def resources(locale, resource_ids):
         for root in settings.FLUENT_PATHS:
-            resources = []
-            for resource_id in resource_ids:
-                path = root.joinpath(locale, resource_id)
-                if not path.is_file():
-                    continue
-
-                with path.open(encoding='utf-8') as ftl_file:
-                    resources.append(FluentResource(ftl_file.read()))
+            resources = load_fluent_resources(root, locale, resource_ids)
             if resources:
-                if locale != 'en':
-                    path = settings.FLUENT_LOCAL_PATH.joinpath('en', 'brands.ftl')
-                    with path.open(encoding='utf-8') as ftl_file:
-                        resources.append(FluentResource(ftl_file.read()))
-
                 yield resources
 
 
@@ -156,6 +144,29 @@ def l10nize(f):
     return inner
 
 
+@memoize
+def load_fluent_resources(root, locale, resource_ids):
+    resources = []
+    for resource_id in resource_ids:
+        path = root.joinpath(locale, resource_id)
+        if not path.is_file():
+            continue
+
+        resources.append(load_fluent_file(path))
+    if resources:
+        if locale != 'en':
+            path = settings.FLUENT_LOCAL_PATH.joinpath('en', 'brands.ftl')
+            resources.append(load_fluent_file(path))
+
+    return resources
+
+
+@memoize
+def load_fluent_file(path):
+    with path.open(encoding='utf-8') as ftl_file:
+        return FluentResource(ftl_file.read())
+
+
 def get_metadata_file_path(ftl_file):
     return settings.FLUENT_REPO_PATH.joinpath('metadata', ftl_file).with_suffix('.json')
 
@@ -206,7 +217,6 @@ def ftl_file_is_active(ftl_file, locale=None):
     return locale in get_active_locales(ftl_file)
 
 
-@memoize
 def fluent_l10n(locales, files):
     if isinstance(locales, str):
         locales = [locales]
@@ -216,13 +226,11 @@ def fluent_l10n(locales, files):
     return FluentL10n(locales, files, FluentResourceLoader)
 
 
-@memoize
 def ftl_has_messages(l10n, *message_ids, require_all=True):
     test = all if require_all else any
     return test([l10n.has_message(mid) for mid in message_ids])
 
 
-@memoize
 def translate(l10n, message_id, fallback=None, **kwargs):
     # check the `locale` bundle for the message if we have a fallback defined
     if fallback and not l10n.has_message(message_id):