extensions: more i18n for goat-exercises

4 of the 5 goat-exercise entries in the menu were had strings extracted for translation but the translation was not used. Looking closer it was because they used N_ rather than _. This commit changes it to _ as is already the case for the Javascript version.

Also the tooltip is now translated for all entries as well as Vala's usage description. Didn't touch the help texts unless they already were N_ and thus already possibly translated.

Couldn't get the usage description to work for JavaScript, but that one already had a comment about possibly changing the style to single line in the future for l10n, so that one can wait.

One thing that I'm not sure about is that https://developer.gimp.org/api/3.0/libgimp/method.Procedure.set_documentation.html suggests to use N_ for blurbs, but when it was used, we had untranslated tooltips. Is this suggestion just for plug-ins not part of core? Looking in the sources I mostly see just "_" for other plug-ins.