Currently, as part of repacks we bundle only one language for the installer/uninstaller.

In order to switch to a different shipping model (one build + language packs), we would need to support multiple languages in the installer/uninstaller.

This bug is filed mostly to explicitly track a dependency for the work needed to get rid of repacks. There's plenty of considerations/choices to make, e.g. should there be a language switcher, should we match the locale used in Firefox or the OS, etc.

Ideally, we should also move away from using "fake" .properties[1], and use Fluent instead for these strings.

[1] We use a different format for placeables than actual properties, access keys are marked with ampersand.

Connecting some tickets here: we have some older build-focused tickets rooted at Bug 1358824. One part of this work, but focusing on the stub installer, is Bug 1715615.