Add translation keys for download chart context menu tooltips

[graphieros]

Tooltip labels for the chart's context menu are not yet translated.

English text:

• 'Download CSV'
• 'Download PNG'
• 'Download SVG'
• 'Toggle annotator'

Translation keys can then be added in PackageDownloadAnalytics.vue, @config.userOptions.buttonTitles:

userOptions: {
  buttons: {...},
  buttonTitles: {
    csv: 'Download CSV',
    png: 'Download PNG',
    svg: 'Download SVG',
    annotator: 'Toggle annotator'
  }
}

[graphieros]

Translation keys are added in en & fr locales, and plugged to the chart configuration.

This issue remains open for other missing translations