Add localization/internationalization support

[kblaschke]

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Currently, all text in the application is hard-coded in English. To make the software accessible to a more international audience, which might not be proficient in English, it is key to provide localized texts depending on the OS/user's locale settings.

Currently, the application doesn't use any framework/library supporting translation of any strings. To make this happen, the following steps need to be taken:

• Add a library supporting string localization. GNU GetText is probably the most compatible choice. It uses .po files, which are supported by a wide variety of translation tools.
• Move all translatable strings into the default English translation file and use it.
• Source more translations from the community over time, ideally from native speakers of each language.