Refine German translations and update non-localized UI elements

[paulbauriegel]

Refine German translations and update non-localized UI elements

• Corrected & added a few German translation in argilla-frontend/translation/de.js
• Added a language aware placeholder to Label selection (LabelSelection.component.vue & EntityLabelSelection.component.vue)
  • $t('spanAnnotation.searchLabels') for filtering Labels in the span Annotation
• The relatives dates in the created & updated field in the Dataset Overview page where hardcoded to English
  • BaseDate.vue -> timeAgo uses now this.$i18n.locale
• The argilla-frontend/docs/snippets/start_page.md sample code names the default workspace argilla even though the server defaults to the default workspace name default so I changed it.
• Set lang in app.vue dynamic to the actual language (important for the screenreader)

Type of change

• Improvement (change adding some improvement to an existing functionality)

How Has This Been Tested
Local front-end build.

Checklist

• I added relevant documentation
• I followed the style guidelines of this project
• I did a self-review of my code
• I made corresponding changes to the documentation
• I confirm My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• I have added relevant notes to the CHANGELOG.md file (See https://keepachangelog.com/)

[leiyre]

Thanks a lot, @paulbauriegel!!