minor #12949 [Translation] Added tip about the incompatibility of % character (welcoMattic)

This PR was submitted for the master branch but it was squashed and merged into the 4.3 branch instead (closes #12949).

Discussion
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[Translation] Added tip about the incompatibility of % character

With the ICU MessageFormat syntax, the character `%` is not valid anymore in parameter keys. It throws a `U_PATTERN_SYNTAX_ERROR` exception

This fix must be backported on all versions from 4.2 included, since the introduction of IntlMessageFormatter

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Commits
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079ff73 [Translation] Added tip about the incompatibility of % character

Author: OskarStark

translation/message_format.rst

@@ -70,6 +70,11 @@ The basic usage of the MessageFormat allows you to use placeholders (called
             'say_hello' => "Hello {name}!",
         ];
 
+
+.. caution::
+
+    With the previous format, placeholders were often named between ``%``. This character is no longer valid with the ICU MessageFormat syntax. Be careful to rename your parameters.
+
 Everything within the curly braces (``{...}``) is processed by the formatter
 and replaced by its placeholder::