.Net: refactor : SummarizeConversation (microsoft#6719)

Maintain consistency in language summarization

### Motivation and Context

  1. Why is this change required?
This change is required to address the issue of inconsistent language
summarization. The previous implementation
defaulted to English summaries regardless of the original language of
the conversation, which could lead to confusion and
      loss of  meaning.
  2. What problem does it solve?
It solves the problem where non-English conversations were incorrectly
summarized in English, leading to potential
     misinterpretation of the summarized content.
  3. What scenario does it contribute to?
This change contributes to scenarios where users engage in conversations
in various languages and expect summaries in
the same language, enhancing the user experience and accuracy of the
summaries.
  4. If it fixes an open issue, please link to the issue here.

### Description
This change contributes to scenarios where users engage in conversations
in various languages and expect summaries in the same language,
enhancing the user experience and accuracy of the summaries.

### Contribution Checklist

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- [x] The code builds clean without any errors or warnings
- [x] The PR follows the [SK Contribution
Guidelines](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md)
and the [pre-submission formatting
script](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md#development-scripts)
raises no violations
- [x] All unit tests pass, and I have added new tests where possible
- [x] I didn't break anyone 😄

dotnet/src/Plugins/Plugins.Core/PromptFunctionConstants.cs

@@ -10,7 +10,7 @@ internal static class PromptFunctionConstants
 
 END CONTENT TO SUMMARIZE.
 
-Summarize the conversation in 'CONTENT TO SUMMARIZE', identifying main points of discussion and any conclusions that were reached.
+Summarize the conversation in 'CONTENT TO SUMMARIZE', identifying main points of discussion and any conclusions that were reached, in the language that best fits the content.
 Do not incorporate other general knowledge.
 Summary is in plain text, in complete sentences, with no markup or tags.