Different translation plugins use different methods for storing the translations, and these methods have varying compatibility with how Relevanssi works with the posts.
Best compatibility
- Polylang. Relevanssi has good built-in support for Polylang. This is the best translation tool that causes the least trouble.
- WPML. Good built-in support, almost as good as Polylang. WPML is more prone to causing weird edge case problems.
- MultilingualPress. Uses separate multisite sites for each language. This works well with Relevanssi, making language filtering automatic. With Relevanssi Premium, you can do multisite searching to include posts in different languages.
Not compatible
- TranslatePress. The translated strings are stored in a separate database, and are invisible for Relevanssi. It is possible to make Relevanssi index the translated strings, but then Relevanssi has no way to index different languages separately. You can’t restrict the search to a specific language; all languages are mixed together.
- GTranslate. The translations are not stored locally at your site, so Relevanssi can’t access them.