common: Fix translation loading, use all paths

[digitalcircuit]

In short

• Fix QTranslator loading Qt translations on Linux
  • Specify directory as directory in load(), not as prefix
  • Fixes loading Qt (not Quassel) translations on Ubuntu 20.04 LTS
• Load translations from all available translation data directories
  • Fixes failure to load translations on Windows due to Qt translation install path
  • Always includes internal resource when present (:/i18n/)

Criteria	Rank	Reason
Impact	★★★ 3/3	Fixes missing translations on Windows
Risk	★★☆ 2/3	Modifies translation search/load code, macOS untested
Intrusiveness	★☆☆ 1/3	Small set of changes, shouldn't interfere with other pull requests

Details

As of 0.14.0, the Windows installer for Quassel places the Qt translations into C:\Program Files\Quassel IRC\translations folder, while Quassel's own translations are bundled internally in :/i18n/.

Before this change, Quassel would only try to load translations from one possible data path, missing the bundled translations in :/i18n/.

After this change, Quassel tries to load translations from all available data paths, including the bundled translations when present.

Testing

Steps

1. Build and install Quassel
   • On Windows, download the Windows.zip artifact from GitHub Actions
2. Open the configuration window
   • Go to Settings → Configure Quassel…
   • Or, press F7
3. Expand the Language: drop-down, check that expected languages are present
4. Select a different language, apply
5. Restart Quassel
6. Check if the language applied successfully

Before

Linux

Qt translations fail to load according to the false return status of qtTranslator->load(…) with QLibraryInfo::location(QLibraryInfo::TranslationsPath).

Windows

All translations fail to load.

The Language: setting in the configuration window displays <Untranslated>, <System Default>, and C as the only options.

After

Linux

Qt translations successfully load according to the true return status of qtTranslator->load(…) with QLibraryInfo::location(QLibraryInfo::TranslationsPath).

NOTE: For unknown reasons, even when Qt claims the translation loaded successfully, system strings (e.g. the Cancel button) does not get translated. Quassel's strings change successfully. This is not a regression as the same issue happens on the master branch.

Windows

Translations load and apply to both Quassel's strings and Qt's system strings (e.g. the Cancel button).

The Language: setting in the configuration window displays all the expected language options.

The Debug Log window now prints the following debug message:

Translation paths: "C:/Program Files/Quassel IRC\translations/", ":/i18n/", with Qt fallback: "C:/Program Files/Quassel IRC/translations"