`wp_update_plugins()` and `wp_update_themes()` are both hooked into `upgrader_process_complete` with priority 10. But at this stage the caches in `get_plugins()`, `search_theme_directories()`, and `wp_get_themes()` aren't refreshed yet so both functions couldn't fetch any translations for the new plugin/theme.
To reset the caches, `wp_clean_themes_cache()` and `wp_clean_plugins_cache()` are now hooked into `upgrader_process_complete` with priority 9.
This is a follow-up to [34751].
See #34029.
Fixes#36383.
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