Fallback formatting fails when key is not defined in fallback translation

[tastendruecker]

Reporting a bug?

The behaviour described in https://kazupon.github.io/vue-i18n/guide/fallback.html#fallback-interpolation does not work, at least with vue-i18n v9.10.2. One can not use the key as a template because included placeholder/parameters will not be replaced when a key and it's returned translation are identical, even if formatFallbackMessages is set to true.

Expected behavior

If formatFallbackMessages is set to true untranslated keys (i.e. key and its translation are identical) should have their parameters replaced.

Reproduction

Simply try the code from https://kazupon.github.io/vue-i18n/guide/fallback.html#fallback-interpolation

System Info

System:
    OS: Linux 5.4 Debian GNU/Linux 11 (bullseye) 11 (bullseye)
    CPU: (8) x64 Intel(R) Core(TM) i7-8565U CPU @ 1.80GHz
    Memory: 10.41 GB / 31.03 GB
    Container: Yes
    Shell: 5.1.4 - /bin/bash
  Binaries:
    Node: 20.11.1 - ~/.nvm/versions/node/v20.11.1/bin/node
    Yarn: 1.22.21 - /usr/bin/yarn
    npm: 10.2.4 - ~/.nvm/versions/node/v20.11.1/bin/npm
  npmPackages:
    @vue/cli-plugin-babel: ^5.0.8 => 5.0.8 
    @vue/cli-plugin-eslint: ^5.0.8 => 5.0.8 
    @vue/cli-plugin-vuex: ^5.0.8 => 5.0.8 
    @vue/cli-service: ^5.0.8 => 5.0.8 
    @vue/compiler-sfc: ^3.4.21 => 3.4.21 
    @vue/eslint-config-prettier: ^9.0.0 => 9.0.0 
    @vue/test-utils: ^2.4.5 => 2.4.5 
    vue: ^3.4.21 => 3.4.21 
    vue-axios: ^3.5.2 => 3.5.2 
    vue-content-loader: ^2.0.1 => 2.0.1 
    vue-directive-tooltip: ^1.6.3 => 1.6.3 
    vue-i18n: ^9.10.2 => 9.10.2 
    vue-lazyload: ^3.0.0 => 3.0.0 
    vue-loader: ^17.4.2 => 17.4.2 
    vue3-toastify: ^0.2.1 => 0.2.1 
    vuex: ^4.1.0 => 4.1.0 
    vuex-persist: ^3.1.3 => 3.1.3 
    vuex4-i18n: ^4.0.4 => 4.0.4

Screenshot

No response

Additional context

No response

Validations

• Read the Contributing Guidelines
• Read the Documentation
• Check that there isn't already an issue that reports the same bug to avoid creating a duplicate.
• Check that this is a concrete bug. For Q&A open a GitHub Discussion
• The provided reproduction is a minimal reproducible example of the bug.

[simondaigre]

Same issue with vue-i18n v10.