autofill: send current language in runtime config

[sjbarag]

ℹ️ This PR replaces #752. There's a small amount of extra context in that PR, based on my original diff ℹ️

Required:

Task/Issue URL: https://app.asana.com/0/11984721910118/1206933877192910/f
iOS PR: N/A
macOS PR: N/A
What kind of version bump will this require?: Patch. This is backwards-compatible, and the autofill codebase falls back to 'en' when it doesn't receive a language.

Optional:

Tech Design URL: N/A
CC: @amddg44 @shakyShane

Description:
The HTML autofill UI will be translated shortly, but doing so is only possible once that UI knows which language to use. Send the current locale's two-character language code as part of the privacy config (aka 'runtime config' in the autofill codebase).

This is a backwards-compatible change, since 'language' is currently an optional input to the autofill HTML pages.

Steps to test this PR:

1. Use autofill from l10n: add basic localization support to device abstraction duckduckgo-autofill#541 + l10n: extract remaining strings to prepare for localization duckduckgo-autofill#558
2. Use BSK from this PR
3. (Optional) Apply this patch to force a pseudolocale:

diff --git a/Sources/BrowserServicesKit/ContentScopeScript/ContentScopeUserScript.swift b/Sources/BrowserServicesKit/ContentScopeScript/ContentScopeUserScript.swift
index 87beab7e..7e1530ec 100644
--- a/Sources/BrowserServicesKit/ContentScopeScript/ContentScopeUserScript.swift
+++ b/Sources/BrowserServicesKit/ContentScopeScript/ContentScopeUserScript.swift
@@ -34,7 +34,7 @@ public final class ContentScopeProperties: Encodable {
     public init(gpcEnabled: Bool, sessionKey: String, featureToggles: ContentScopeFeatureToggles) {
         self.globalPrivacyControlValue = gpcEnabled
         self.sessionKey = sessionKey
-        languageCode = Locale.current.languageCode ?? "en"
+        languageCode = "xa"
         features = [
             "autofill": ContentScopeFeature(featureToggles: featureToggles)
         ]

4. Build and launch the macOS app
5. Load any webpage with a signup form (I typically use IMDB)
6. Click on Dax
7. Confirm translation of the "Block email trackers" text under your personal Duck address:
   1. If you didn't apply the pseudolocale patch, you should still see English (the default in Autofill):
      
   2. If you did apply the pseudolocale patch, you should see text that almost looks like English if you squint, but also has some repeated l33tspeak-style symbols:
      

OS Testing:

• iOS 14 — No changes, if I understand correctly.
• iOS 15 — No changes, if I understand correctly.
• iOS 16 — No changes, if I understand correctly.
• macOS 10.15 — I don't have 10.15 available
• macOS 11 — I don't have 11 available
• macOS 12 — Looking into it
• macOS 14

---

Internal references:

Software Engineering Expectations
Technical Design Template

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• To see the specific tasks where the Asana app for GitHub is being used, see below:
  • https://app.asana.com/0/0/1206933877192910

[amddg44]

LGTM! 👏