Simply recommend UTF-8 without a BOM for .isl files.

ISHelp/isetup.xml

@@ -2154,7 +2154,7 @@ Name: "nl"; MessagesFile: "compiler:Languages\Dutch.isl"
 <p>Specifies the name(s) of the .isl file(s) to read the default messages from. The file(s) must be located in your installation's <link topic="sourcedirectorynotes">source directory</link> when running the compiler, unless a fully qualified pathname is specified or the pathname is prefixed by "compiler:", in which case it looks for the file in the compiler directory.</p>
 <p>Each message file may contain a <link topic="langoptionssection">[LangOptions] section</link>, a <link topic="messagessection">[Messages] section</link>, and a <link topic="custommessagessection">[CustomMessages] section.</link></p>
 <p>When multiple files are specified, they are read in the order they are specified, thus the last message file overrides any language options or messages from previous files. Any language options or messages in the main script override the ones from message files.</p>
-<p>If an Unicode file is used, it must be UTF-8 encoded.</p>
+<p>The file is recommended to be UTF-8 encoded, without a BOM.</p>
 <examples>
 <pre>
 MessagesFile: "compiler:Dutch.isl"