How OpenSEO Manages Multi-Language Locations: Switzerland de/fr/it Implementation Guide

OpenSEO handles multi-language locations by storing a catalog of DataForSEO location codes paired with default languages and explicit multi-language mappings, enabling validated language selection for countries like Switzerland that support German, French, and Italian.

OpenSEO implements a robust localization architecture to manage SEO data retrieval across regions with multiple official languages. The system maintains type-safe registries in TypeScript that define which languages are valid for each numeric location code, ensuring that API requests to DataForSEO only contain supported locale combinations and preventing costly validation errors.

Location and Language Architecture in OpenSEO

The foundation of OpenSEO’s multi-language support rests on three coordinated data structures defined in src/shared/keyword-locations.ts.

First, each country maps to a numeric code (e.g., 2756 for Switzerland). Second, the LOCATION_LANGUAGE constant associates every location code with its primary default language. Third, the MULTI_LANGUAGE_LOCATIONS registry explicitly enumerates countries that support additional languages beyond their default.

For Switzerland, this registry defines the available language codes as an array:

2756: ["de", "fr", "it"], // Switzerland

This approach allows the system to distinguish between single-language countries (which rely solely on the default language mapping) and multi-language regions that require explicit user selection capabilities.

Resolving Language Options for UI Components

When rendering language pickers in the frontend, OpenSEO calls getLanguageOptions(locationCode) to build a filtered list of valid choices. This function checks the MULTI_LANGUAGE_LOCATIONS map first; if the location exists in the registry, it uses those codes, otherwise falling back to the single default language.

Located around line 814 in src/shared/keyword-locations.ts, the implementation uses a Set to deduplicate codes before filtering the global SERP_LANGUAGE_OPTIONS:

export function getLanguageOptions(locationCode: number) {
  const codes = new Set(
    MULTI_LANGUAGE_LOCATIONS[locationCode] ?? [getLanguageCode(locationCode)],
  );
  return SERP_LANGUAGE_OPTIONS.filter((language) => codes.has(language.code));
}

For Switzerland (code 2756), this returns German, French, and Italian options, while a country like the United States would return only English.

Market Resolution and Validation

Before issuing requests to the DataForSEO SERP API, OpenSEO must resolve which language code to transmit. The resolveMarket function handles this logic by accepting user arguments and project defaults, automatically selecting the location’s default language when none is specified:

export function resolveMarket(args, project) {
  const locationCode = args.locationCode ?? project.locationCode;
  const languageCode =
    args.languageCode ??
    (locationCode === project.locationCode
      ? project.languageCode
      : getLanguageCode(locationCode));
  return { locationCode, languageCode };
}

For validation, assertLanguageForLocation in src/server/lib/market.ts (around line 34) guards against invalid combinations. It utilizes isLanguageServedForLocation, which references the same MULTI_LANGUAGE_LOCATIONS data to verify that a requested language is actually served for the target location before any external API call occurs.

Keyword Data API Language Handling

The DataForSEO Labs endpoints (keyword data) impose stricter language constraints than the SERP API, accepting only a country’s supported languages. To handle this, resolveKeywordDataLanguage validates the requested language against getLanguageOptions and falls back to the location’s default when an unsupported language is supplied:

return getLanguageOptions(locationCode).some(o => o.code === languageCode)
  ? languageCode
  : getLanguageCode(locationCode);

This ensures that requests to Labs-based tools never trigger "Invalid Field: ‘language_code’" errors, automatically correcting selections like Spanish (es) for Switzerland to German (de).

Practical Implementation Examples

The following patterns demonstrate how OpenSEO’s multi-language utilities function in practice:

import { getLanguageOptions, resolveMarket, resolveKeywordDataLanguage } from '@/shared/keyword-locations';

// 1. Retrieve supported languages for Switzerland
const swissLangs = getLanguageOptions(2756);
console.log(swissLangs.map(l => l.label)); // → ["German", "French", "Italian"]

// 2. Resolve market when user selects location without specifying language
const market = resolveMarket(
  { locationCode: 2756 }, 
  { locationCode: 2144, languageCode: 'en' }
);
// market = { locationCode: 2756, languageCode: 'de' } – defaults to German

// 3. Validate language for Labs keyword-data API with fallback
const validLang = resolveKeywordDataLanguage(2756, 'fr');
console.log(validLang); // → "fr"

const fallbackLang = resolveKeywordDataLanguage(2756, 'es');
console.log(fallbackLang); // → "de" (fallback to default)

Summary

• Registry-based architecture: OpenSEO stores location definitions in src/shared/keyword-locations.ts, separating default languages from explicit multi-language mappings.
• Switzerland support: Location code 2756 explicitly lists ["de", "fr", "it"] in the MULTI_LANGUAGE_LOCATIONS constant.
• Dynamic UI generation: The getLanguageOptions function filters available languages based on the registry, presenting only valid choices to users.
• Early validation: assertLanguageForLocation in src/server/lib/market.ts prevents invalid API requests before they reach DataForSEO.
• Intelligent fallback: resolveMarket and resolveKeywordDataLanguage automatically default to the location’s primary language when unspecified or invalid.

Frequently Asked Questions

Which countries does OpenSEO support for multi-language SEO targeting?

OpenSEO supports any country defined in the DataForSEO location catalog, with explicit multi-language definitions for regions like Switzerland (2756), Belgium, and Canada. The MULTI_LANGUAGE_LOCATIONS constant in src/shared/keyword-locations.ts enumerates countries where multiple language options are exposed in the UI, while all other locations default to their single primary language defined in LOCATION_LANGUAGE.

How does OpenSEO prevent invalid language codes in DataForSEO API calls?

Before transmitting requests, OpenSEO calls assertLanguageForLocation which utilizes isLanguageServedForLocation to check the requested language against the MULTI_LANGUAGE_LOCATIONS registry. If the combination is invalid, the system throws an error or falls back to the default language (in the case of Labs endpoints via resolveKeywordDataLanguage), preventing "Invalid Field" errors from the DataForSEO API.

What happens when a user selects a language not supported for a specific location?

For SERP API calls, OpenSEO validates the language early and rejects unsupported combinations. For Labs keyword-data endpoints, the system automatically falls back to the location’s default language. For example, requesting Spanish (es) for Switzerland (2756) results in the system defaulting to German (de) to ensure the API call succeeds.

How does the system handle language selection when a user changes locations mid-project?

The resolveMarket function in src/shared/keyword-locations.ts compares the requested locationCode against the project's current location. When they differ, it automatically resolves the language to the new location’s default via getLanguageCode, ensuring that switching from a project based in the UK to one targeting Switzerland automatically adopts German unless explicitly overridden.

{
  "mcpServers": {
    "instagit": {
      "command": "npx",
      "args": ["-y", "instagit@latest"]
    }
  }
}