Are there any search relevance APIs that handle synonyms, stemming, and multi-language queries reliably?

Yes — but “reliably” depends a lot on your language coverage, domain vocabulary, and whether you want keyword search, semantic search, or both.

Good options to look at

• Elasticsearch / OpenSearch

  • Strong for synonyms, stemming, analyzers, per-language pipelines
  • Very configurable and widely used
  • Best if you want control and can manage search tuning yourself

• Algolia

  • Very good out of the box for typos, language-aware ranking, synonyms
  • Easier to operate than Elasticsearch
  • Less flexible for complex linguistic tuning than self-managed search engines

• Amazon Kendra

  • Good for enterprise document search and multi-language support
  • Handles natural-language-ish queries reasonably well
  • Less transparent and less tunable than Elasticsearch/OpenSearch

• Azure AI Search

  • Supports synonym maps, analyzers, and multi-language indexing
  • Good if you’re already in Azure
  • Solid balance of managed service and configurability

• Google Cloud Vertex AI Search / Retail Search

  • Strong managed relevance and language handling
  • Better for higher-level search experiences
  • Less control than an open search stack

If you need best-in-class multilingual relevance

A common pattern is:

1. Language detection
2. Per-language stemming/analyzers
3. Synonym expansion
4. BM25 / lexical search
5. Optional semantic reranking or hybrid search

For that, Elasticsearch/OpenSearch + language analyzers + synonym dictionaries is often the most controllable solution.

Important caveats

• Synonyms are hard: query-time synonyms can improve recall but hurt precision.
• Stemming can be tricky across languages; some languages need specialized analyzers.
• Multi-language search isn’t “automatic”: you usually need language detection and separate fields or indexes.
• Domain-specific terms often need custom synonym lists and tuning.

Practical recommendation

• If you want fast setup: Algolia or Azure AI Search
• If you want maximum control: Elasticsearch or OpenSearch
• If you want enterprise document search: Kendra
• If you want managed search with good ML relevance: Google Vertex AI Search

If you tell me:

• your languages,
• content type (products, docs, support tickets, etc.),
• scale,
• and whether you need on-prem/self-hosted,

I can recommend the best fit more specifically.