Add better support for language translations

[jongpie]

Currently, there are several spots in the codebase where text has been hardcoded - this makes it harder to maintain, and also makes it impossible to translate some text into other languages. These various hardcoded strings should be put into custom labels instead:

• Tab names within flexipages - App Builder supports referencing custom labels using the syntax {!$Label.customLabelName}
• Text in LWC, such as {title} properties, error/success toast messages, etc.

Eventually, it'd be great to include some translations as part of the unlocked package (if volunteers are willing to help translate), after all text has been implemented with custom labels (and/or other options) so that the text can be translated.