langtools/src/jdk.compiler/share/classes/com/sun/tools/javac/resources/javac_ja.properties:
　javac.opt.Xbootclasspath.p=ブートストラップ・クラス・パスに付加する

Original English value is "Prepend to the bootstrap class path".
"付加する" is used as translation of "Prepend", but its meaning is unclear. It doesn't specify where a given path is added to. Something which means "at the front" is necessary.