Standardize localization with GNU gettext format

[nightm4re94]

In order to make translators' lives easier, we could apply the GNU gettext portable object format to our game localization files. Then, they could be edited more conveniently with tools such as poedit. I think the PO format is superior to XLIFF in terms of readability by humans, which is why I wouldn't consider using XLIFF.

We could also have a look at how the mini2Dx java game library handles gettext PO files. Conveniently, the library provides a Gradle plugin to automatically parse source files and generate templates.

Read more about the PO format and its specifications:
http://pology.nedohodnik.net/doc/user/en_US/ch-poformat.html
https://www.drupal.org/node/1814954

[steffen-wilke]

https://docs.weblate.org/en/weblate-3.10/formats.html#gettext