i18n: fix blank error messages when untranslated

If a message has a translation in any locale, but not all locales, then
translatable_messages::translate with that message can return an empty
string (instead of returning a translated string or the original
untranslated string).

Fix this bug in translatable_messages::translate. Also, add a test suite
which exhaustively checks every translation. (I wrote this test suite
for another purpose, then stumbled upon this bug. Hurray!)

Author: strager

docs/CHANGELOG.md

@@ -6,6 +6,13 @@ based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 quick-lint-js' version numbers are arbitrary. quick-lint-js does *not* adhere to
 Semantic Versioning.
 
+## Unreleased
+
+## Fixed
+
+* When using the `de`, `fr_FR`, or `sv_SV` locale, some error messages are no
+  longer blank.
+
 ## 2.4.0 (2022-04-15)
 
 [Downloads](https://c.quick-lint-js.com/releases/2.4.0/)

src/translation.cpp

@@ -131,6 +131,10 @@ const char* translatable_messages::translate(
     const translation_table::mapping_entry& mapping =
         translation_data.mapping_table[mapping_index];
     std::uint32_t string_offset = mapping.string_offsets[this->locale_index_];
+    if (string_offset == 0) {
+      // The string has no translation.
+      return message.c_str();
+    }
     return reinterpret_cast<const char*>(translation_data.string_table +
                                          string_offset);
   } else {

test/CMakeLists.txt

@@ -25,6 +25,7 @@ quick_lint_js_add_executable(
   quick-lint-js/spy-lsp-endpoint-remote.h
   quick-lint-js/spy-lsp-message-parser.h
   quick-lint-js/spy-visitor.h
+  quick-lint-js/test-translation-table-generated.h
   spy-visitor.cpp
   test-array.cpp
   test-assert.cpp