### Godot version

v4.2.dev4.official [549fcce5f]

### System information

Ubuntu 20.04 LTS

### Issue description

See title.

### Steps to reproduce

1. Create a new project
2. Create a csv file that includes the following
```
key,ar
key,placeholder
```
3. Add the generated `.translation` file to Project -> Project Settings... -> Localization
4. In Project -> Project Settings... -> Internationalization -> Locale, set `Test` to `ar`
5. Add a `LineEdit` node and type `-=[]{}`
6. Run the project, and notice that the text is on the right side, but the symbols are incorrectly in LTR (`-=[]{}`).  Correct output is `{}[]=-`
7. Back in the editor, add a single Arabic letter (any will do), and notice that `LineEdit` immediately (and correctly) flips the symbols, and the newly-added Arabic letter, to RTL.

**Notes**
- Identical behavior is seen with `TextEdit`.
- The purpose of steps 2-4 are to simulate an Arabic environment when the project is run, despite `OS.get_locale() == "en"` (or your system's current language).

### Minimal reproduction project

[rtlsymbols.zip](https://github.com/godotengine/godot/files/12525509/rtlsymbols.zip)
#ifndef ICU_STATIC_DATA
	if (!icu_data_loaded) {
		return p_string.to_upper();
	}
#endif
	if (p_string.is_empty()) {
		return p_string;
	}
	const String lang = (p_language.is_empty()) ? TranslationServer::get_singleton()->get_tool_locale() : p_language;
	// Convert to UTF-16.
	Char16String utf16 = p_string.utf16();
	Vector<char16_t> upper;
	UErrorCode err = U_ZERO_ERROR;
	int32_t len = u_strToUpper(nullptr, 0, utf16.get_data(), -1, lang.arr);
	ERR_FAIL_COND_V_MSG(err != U_BUFFER_OVERFLOW_ERROR, p_string, u_errorName(err));
	upper.resize(len);
	err = U_ZERO_ERROR;
	u_strToUpper(upper.ptrw(), len, utf16.get_data(), -1, lang.arr);
	ERR_FAIL_COND_V_MSG(U_FAILURE(err), p_string, u_errorName(err));
	// Convert back to UTF-32.
	return String::utf16(upper.ptr(), len);
}
String TextServerAdvanced::_string_to_lower(const String &p_string, const String &p_language) const {