It appears that the appstream project no longer supports
`translatable=no` properties, and gettext extract the
`translatable=no` marked strings as translatable.

I opened an issue to inform about the situation, but `translatable=no`
properties are not accepted by developers. You can find the issue
here: ximion/appstream#623

Also correct translatable usage.

**Please test your script or string extraction process
before merging this PR.**

> In MetaInfo files, each individual paragraph of a description
> (or enumerated entry) is translated individually, however,
> you can only exclude the complete block from being translated
> by adding `translate="no"` to the description element.

Source: https://freedesktop.org/software/appstream/docs/sect-Quickstart-Translation.html