Multi-site and multilingual 

Several sites in one instance 

Create one task per site, with startPageId on that site's root page:

Task "Similarities – Main site"      startPageId: 1    languageId: -1
Task "Similarities – Campaign site"  startPageId: 85   languageId: -1

The task resolves the site from startPageId and only ever walks that site's page tree, so tasks cannot contaminate each other. Extra tasks on subtrees of a site are allowed too — for instance a daily task on a fast-moving news section and a weekly one on the rest. Each rewrites only its own scope_page_id.

What is scoped per site automatically:

• Frontend display. The lookup filters on the current page's site, so a page of one site can never be suggested on another. No configuration needed.
• Cache invalidation. Analysis cache entries are tagged site_<rootPageId>, so editing content on one site does not invalidate another's analysis.
• Backend module. Permission-scoped per site — see Access control.

What is not scoped, and needs your attention:

• TypoScript. The extension's constants and setup are loaded instance-wide by ext_localconf.php, so every setting is shared until a site's root template overrides it. This is why the Bootstrap Package integration (Bootstrap Package) must be enabled per site and never globally.
• Extension configuration and the log file, which are instance-wide by nature.

A multilingual site 

Language handling is driven by the site configuration. Each language's locale gives the language code used for stop words, stemming and vectorisation:

# config/sites/main/config.yaml
languages:
  -
    languageId: 0
    title: 'English'
    locale: 'en_US.UTF-8'      # → analysed as "en"
  -
    languageId: 1
    title: 'Deutsch'
    locale: 'de_DE.UTF-8'      # → analysed as "de"

One task is enough. A multilingual site has a single root page and several languages entries; with the default languageId = -1 the task iterates over all of them in one run:

Task "Similarities – Main site"
  startPageId:  1     # the site root — one per SITE, not per language
  languageId:   -1    # all languages of this site
  qualityLevel: 0.3

Do not create one task per language pointing at different page UIDs: that pattern belongs to multi-site setups, where each site has its own root page.

Split by language only when a language needs a different quality level. Both tasks then target the same root page with an explicit language ID:

Task 1  startPageId: 1   languageId: 0   qualityLevel: 0.3
Task 2  startPageId: 1   languageId: 1   qualityLevel: 0.25

Rows are keyed by sys_language_uid on top of the site and the scope, so the two tasks never collide.

Languages never mix 

The language boundary is enforced three times over, so mixed-language suggestions are not something you have to configure away:

1. Pages are collected per language by the task.
2. Two pages whose detected language codes differ score 0.0 and are not stored.
3. The frontend query filters on sys_language_uid and re-checks the language of each candidate page record.

If suggestions really do cross a language, the cause is almost always a missing or malformed locale in the site configuration — see Troubleshooting.

Per-site templates 

Override the plugin template in the site's root template, using index 10 or above:

plugin.tx_semanticsuggestion_suggestions.view {
    templateRootPaths.10 = EXT:my_sitepackage/Resources/Private/Templates/
}

See Overriding the template.