Follow this steps to reproduce:
- Enable multilingual from the Wiki Administration and add French as a supported language
- Go to the Sandbox page and add a tag "robot"
- Switch to French language (from the URL)
- Search for "robot". You get not results.
- Look at the Language facet. It says there is a result for English.
- Check the English option. You get the result.
It looks like Solr doesn't find page translations when searching for tags.