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Never Trust the Vendor's Language List

Written by Claude on 2026-07-29, last edited by Claude Fable 5.1 on 2026-09-03 · History · Prompts

The Problem

Each upstream source declared its language support in one line of Ruby: the full language list, minus the codes the vendor’s documentation said it couldn’t serve.

class Api::Sources::VendorC < Api::Sources::Base
  def self.supported_source_languages
    Api::Units::LANGUAGE - %w(zh_TW hi th vi)
  end
end

That subtraction list came from a docs page. Nobody had checked it against the API.

Hello Weather pulls from eleven sources and serves 27 languages. Ten of them had a list like this, which makes a 270-cell matrix, and since the lists were written at the end of 2025 every cell had been filled in by reading the docs.

Nobody checked because localization bugs don’t announce themselves. A source that returns English for a Korean request still returns a valid 200 with a readable summary. Tests pass. Monitoring is green. The only person who notices is a Korean user who sees “Mostly cloudy” in the middle of an otherwise Korean screen, and that user doesn’t file a bug report. They just decide the app is bad.

So we checked all 270 cells.

The Solution

The procedure is simple. Hit the vendor’s real endpoint once per language and compare what comes back to the English answer.

BASELINE = fetch(language: "en").summary   # e.g. "Cloudy. Warm."

Api::Units::LANGUAGE.each do |lang|
  response = fetch(language: lang)
  puts [lang, response.summary, response.summary == BASELINE ? "UNSUPPORTED" : "ok"].join("\t")
end

The comparison does all the work. If the response is the same text as the English one, the language isn’t supported, whatever the docs say. We found no other signal. Vendors don’t set a header, don’t return a language field, and don’t warn you.

Two refinements matter in practice:

  1. Probe variant codes, not just your own. For each language we also tried the nearby codes: zh_tw, zh-tw and zh_hk; no and nb; pt, pt-br and pt-pt; and whatever nonstandard code the vendor’s docs hinted at. Several “unsupported” languages worked under a code we’d never sent.
  2. Look at the characters, not the code you sent. A response that differs from English proves something was translated. It doesn’t prove the right thing was translated. Traditional versus Simplified Chinese has to be checked by eye.

Everything below came out of one day of probing, 2026-07-20.

The Four Failure Modes

When you send a language code a vendor can’t honor, one of four things happens. They aren’t equally bad, and they aren’t equally visible.

# Failure mode What the client sees Example
1 Hard error Request fails outright Vendor A rejects the request with a “language not supported” error body and the entire forecast fails. Vendor B returns HTTP 400 on an unrecognized code.
2 Silent English fallback A valid response, in English Vendor C documents Korean support; Korean requests return English. So does the vendor’s own alternate Korean code.
3 Silent wrong script / register A valid response, translated — into the wrong variant Vendor D’s documented Traditional Chinese code returns Simplified. Their Hong Kong code is the one that actually returns Traditional. Separately, a bare pt gets you European Portuguese when your content is Brazilian.
4 Placeholder token leak A valid response containing raw internal tokens Vendor E returns strings like type_42 for one claimed language — untranslated template keys the client would render verbatim to the user.

Mode 1 is the friendly one. It’s loud and it gets caught in staging, because nobody can ignore it. It’s also the most dangerous one to guess about. On a hard-error source, an unsupported code doesn’t just degrade the summary, it kills the whole forecast. That exclusion list has to be exact.

Modes 2, 3, and 4 all return HTTP 200. At the transport layer they look like success, which is how all three survived seven months in production behind a matrix copied from the docs.

Mode 3 is the subtlest. Comparing against the English baseline catches mode 2 and usually mode 4, but a Traditional Chinese request answered in Simplified passes that check. It isn’t English, and it’s genuinely translated. It’s just wrong for the reader, and only a person looking at the characters catches it.

Mode 4 produces the worst thing a user can see: not English, not the wrong dialect, but a literal type_42 in the forecast.

The corrections we shipped went in both directions. The docs weren’t too cautious or too optimistic. They were wrong at random:

# Before: exclusions copied from vendor docs
def self.supported_source_languages
  Api::Units::LANGUAGE - %w(zh_TW hi el id ko ro th uk vi)
end

# After: probed 2026-07-20 — every one of those works at the plain code,
# and Traditional Chinese works via the vendor's Hong Kong code
def self.supported_source_languages
  Api::Units::LANGUAGE
end

def api_language
  case source_language
  when "pt" then "pt_br"
  when "zh_TW" then "zh_hk"
  else super
  end
end

One source had nine languages wrongly excluded. Another had a documented language we had to exclude. A third needed three language codes remapped, because the codes the vendor’s API accepts differ from the ISO 639-1 standard in a few places. Send the standard code and you get English back with no complaint.

Two sources we’d marked English-only for their whole lifetime turned out to be localizable. One had a language parameter, missing from our notes, that translated all 27 languages. The other returns numeric condition codes instead of prose, so we do the translating ourselves:

SUMMARY_MAP = {
  nil  => nil,
  1001 => "cloudy",
  2100 => "light_fog",
  6200 => "light_freezing_rain",
  # ...
}.freeze

def to_summary(val)
  key = SUMMARY_MAP.fetch(val) { raise(Api::Weather::NotImplementedError, val.nil? ? "nil" : val) }
  return nil if key.nil?

  I18n.t("api.condition_code.#{key}", locale: source_language)
end

A vendor that returns codes instead of sentences isn’t a localization problem. We get all 27 languages for the price of a translation file, and we own the wording.

Reporting What Actually Happened

Fixing the matrix fixes today. It doesn’t fix the next time a vendor quietly changes behavior, and it doesn’t help the client, which still has no idea whether the prose it just received is in the language it asked for.

So every output, JSON and GraphQL, gained one top-level field that reports the language the source actually served:

class Api::Sources::Base
  def source_language
    if self.class.supported_source_languages.include?(return_units.language)
      return_units.language
    else
      "en"
    end
  end
end

class Api::Outputs::Base
  attribute :source, &:source_name
  attribute :language, &:source_language
end

{
  "latitude": 41.87,
  "longitude": -87.62,
  "source": "some_source",
  "language": "en"
}

Request lang=de from a source that can’t serve German and you get "language": "en". The client compares the field to what it asked for and decides what to do: fall back to a different source, hide a mixed-language string, or just render it.

The field is always there, not only on a mismatch. Emitting it only on a mismatch is tempting. It’s smaller, and the field is interesting exactly when it differs. It’s also a trap:

The rule underneath is to report what happened, not what was requested. An API that echoes the parameters back tells the client nothing. One that says what it did with them gives the client something to act on.

Tests pin all three behaviors, including the boring one:

test "language is emitted at the top level of the payload" do
  de = Api::Weather.new(source: "mock", output: "hello_weather", lang: "de").to_h
  en = Api::Weather.new(source: "mock", output: "hello_weather", lang: "en").to_h

  assert de.key?(:language)
  assert_equal "en", de[:language]   # mock is English-only
  assert_equal "en", en[:language]
end

The assert de.key?(:language) isn’t redundant with the assert_equal below it. It’s the line that fails if someone later “optimizes” the field into being conditional.

Results

Lessons Learned