Use internationalization in my manifest.json of search provider webextension - api

Question

I use internationalization in mymanifest.json of search provider webextension for Firefox, I have not installed a language pack and set intl.locale.requested to "de" .My issue is that some text are showing in "de" language and some are showing in default english language.I can't figure out reasons for the issue.
Below is my manifest.json
I have used installed the language pack for de in Firefox and its fixe everything but I want to know the cause of the issue.
manifest.json
{
"manifest_version": 2,
"name": "__MSG_extensionName__",
"description": "__MSG_extensionDescription__",
"version": "1.1.3",
"applications": {
"gecko": {
"strict_min_version": "57.0"
}
},
"icons": {
"64": "icons/my-icon.png"
},
"permissions": [
"activeTab",
],
"chrome_settings_overrides": {
"search_provider": {
"name": "__MSG_searchEngineName__",
"search_url": "https://www.example.com/do/dsearch?query={searchTerms}&language=__MSG_extensionUrlLanguage__",
"favicon_url": "https://www.example.com/favicon.ico",
"is_default": true
}
},
"background": {
"scripts": ["js/background.js"]
},
"content_scripts": [
{
"matches": ["https://*.example.com/*"],
"css": ["css/content.css"],
"run_at": "document_start"
},
{
"matches": ["https://*.mydomain.com/*"],
"js": ["js/content.js", "js/success.min.js"]
}
],
"default_locale": "en"
}
message.json for de locale
{
"extensionName": {
"message": "example.com — Datenschutz-Suchmaschine",
"description": "Name of the extension."
},
"extensionDescription": {
"message": "Hol dir deine Online-Privatsphäre zurück, mach domain.com zu deiner Suchmaschine.",
"description": "Description of the extension."
},
"extensionUrlLanguage": {
"message": "deutsch",
"description": "Search Engine Language"
},
"searchEngineName": {
"message": "example.com - Deutsch",
"description": "Search Engine Name"
}
}
expected output in addons list extension shows up with text in de language and perform the search with URL https://www.example.com/do/dsearch?query={searchTerms}&language=deutsch
actual results:
expected output in addons list extension shows up with text in de language and perform the search with URL ttps://www.example.com/do/dsearch?query={searchTerms}&language=english