Add translations for the Add Extension modal
Add translations for the Add Extension modal

- Adds three translations for this placeholder dialog.
- Updates the instructions to mention Composer.
- Supports

flarum/flarum-ext-english,pladask/flarum-ext-norwegian-nynorsk,pladask/flarum-ext-norwegian-bokmal,gtugablue/flarum-ext-portuguese,pladask/flarum-ext-norwegian-nynorsk,balkanje/flarum-ext-bosnian1,ogioncz/flarum-ext-czech,FlarumChina/flarum-ext-simplified-chinese-pro,ogioncz/flarum-ext-czech,flarum/english,FlarumChina/flarum-ext-simplified-chinese-pro,pladask/flarum-ext-norwegian-bokmal,raweba/flarum-ext-danish