Language setting reverts to Polish after tab navigation

[hyper-charlie]

Description

Language setting does not persist. When user changes the main language from Polish to English in settings, it displays as English initially. However, when navigating to another tab and returning to settings, the language reverts to Polish.

Steps to Reproduce

1. Open Settings
2. Change main language from Polish to English
3. Language displays as English
4. Navigate to a different tab
5. Return to Settings tab
6. Language has reverted to Polish

Expected Behavior

Language setting should persist across tab navigation and app sessions.

Actual Behavior

Language setting reverts to Polish after navigating away from the settings tab.

Environment

• Platform: macOS (desktop version)
• App Version: 1.0.14
• Build: 57054b4
• OS Version: macOS 26.3.1
• Architecture: aarch64
• Locale: pl-PL

Notes

User did not attempt workarounds to avoid data loss.

[devin-ai-integration]

Investigation Findings

Root Cause

The language setting revert is caused by the interaction between two mechanisms:

1. storeValuesToSettings skips values equal to the schema default — introduced in 43dcacec3 (fix: allow config defaults to be handled by the settings store #4370). In transform.ts:132-134:

   if ("default" in config && value === config.default) {
     continue;
   }

   The default for ai_language is "en" (settings.ts:62). So when the user changes from Polish → English, the value "en" matches the default and is not written to settings.json.

2. applyLanguageDefaults fills in missing values from OS locale on load — in persister.ts:36-38:

   if (language.ai_language == null && defaults.ai_language) {
     language.ai_language = defaults.ai_language;
   }

   This calls detectCommands.getPreferredLanguages() which returns ["pl"] for this user's pl-PL locale. Since "en" was never persisted to the file, on the next load the OS locale (pl) is applied.

The Save→Load Cycle

1. User sets language to English ("en")
2. TinyBase store updates in memory → works fine initially
3. Auto-save triggers: storeToSettings → storeValuesToSettings omits ai_language because "en" === "en" (the default)
4. settings.json is written without ai_language
5. When settings reload (file change event triggers auto-load, or app restart), applyLanguageDefaults sees ai_language is missing and fills it with the OS locale (pl)
6. Language reverts to Polish

The existing test at persister.test.ts:120 actually documents this: delete (expected as any).language.ai_language; — it explicitly expects "en" to be dropped during roundtrip.

Relevant Files

File	Role
apps/desktop/src/store/tinybase/persister/settings/transform.ts	storeValuesToSettings skips default values (line 132)
apps/desktop/src/store/tinybase/persister/settings/persister.ts	applyLanguageDefaults overwrites missing values with OS locale (line 36)
apps/desktop/src/store/tinybase/store/settings.ts	ai_language default is "en" (line 62)
apps/desktop/src/settings/general/index.tsx	Settings form UI
apps/desktop/src/settings/general/main-language.tsx	Main language selector component

Potential Fix Approaches

1. Don't skip ai_language (and spoken_languages) when they equal the default — these fields interact with OS locale detection, so the default-stripping optimization is harmful here. Could add an exception list in storeValuesToSettings, or add a persistAlways flag to the mapping.

2. Change applyLanguageDefaults to only apply when the setting has never been set — e.g., use a separate flag or sentinel value to distinguish "user explicitly chose the default" from "never configured".

3. Remove the default-skipping for all values — simpler but results in a larger settings.json. The optimization was introduced in fix: allow config defaults to be handled by the settings store #4370; reverting just the skip logic in storeValuesToSettings while keeping the rest of that PR's improvements would fix this class of bugs for all settings.

Approach 1 is likely the most targeted fix with fewest side effects.