Bug in UnloadLanguage + fallback strings

I've come across what I believe is a bug in the language unloading behaviour, which may be related to inter-illusion.com/forum/i2-localization...on-missing-on-device . This is a bit complicated to explain, but here goes.

Setup:
- Unity 2017.4.9f1
- I2Loc 2.8.9 f1
- Windows 10
- All Google Sheet behaviour disabled ("Never" check frequency).
- Unload languages at runtime: "Device only" (or "Editor and Device")


On Windows, in the standalone player, the data for each language is written to file by Export_Language_to_Cache during startup. This results in files in Application.temporaryCachePath, which is %TEMP%\[companyname]\[productname] on disk. It is during this save-to-disk process that the fallback string for terms with no translations is also determined (and saved both in memory and in the disk cache). However, the fallback will fail to find other languages if they have already unloaded, which happens only if the cache files already exist.

Here's how this works. The very first time you run the game, the cache files do not exist. This is roughly the flow during startup:

1. Sources are added from the I2Languages asset
2. All langauges are set loaded=true
3. The initial language is set as the current language
4. A call to LoadLanguage is made
5. A call to unload all languages other than the current language is made
- This returns early (without unloading), because there is no cache file yet. Languages will not unload unless there is a cache file. See LanguageSource_Language.UnloadLanguage(), line 283: !PersistentStorage.HasFile(PersistentStorage.eFileType.Temporal, GetSavedLanguageFileName(languageIndex)
6. Languages get saved, and Export_Language_to_Cache is called for each language
- As each language is exported, any missing terms are filled in with fallback values.
- These fallback values are found in other languages, because everything is still loaded
- The current language gets fallback values filled in, both in memory and in the written cache file
7. UnloadLanguage is called again, and this time succeeds.


For any subsequent launch of the standalone player, this happens instead:

1. Sources are added from the I2Languages asset
2. All langauges are set loaded=true
3. The initial language is set as the current language
4. A call to LoadLanguage is made
5. A call to unload all languages other than the current language is made
- This time, the languages ARE unloaded successfully (because a cache file exists on disk).
6. Languages get saved, and Export_Language_to_Cache is called for each language
- As the current language is exported, any missing terms are filled in with fallback values.
- However, the other languages have been unloaded, so fallbacks are no longer available
- Terms with missing translations are now stuck with empty translations
- This is also written to disk. You can see the cache file is updated and all "[i2fb]" strings are removed. In practice, this does not matter because the language is never actually imported from these cache files (with these settings).
7. For completeness, UnloadLanguage is called again, though languages are already unloaded.


Note that the Editor behaves differently, as it clears the cache files from disk whenever you exit Play Mode. These means you can get some unintuitive behaviour if you play in Editor after playing Standalone (because there are cache files when usually they have been deleted), or by playing Standalone after playing in the Editor (because cache files will have been deleted, which puts you into the "first time" scenario above).

I totally understand that this might not be clear! The important bit is that the UnloadLanguage behaviour in step 5 above differs based on whether or not these cache files exist, which in turn affects how fallback translations are determined in step 6.

Great job documenting this! I just ran into the exact same issue (seems to be Windows only) and had temporarily disabled the unload. I came to the forum to report the issue.