实现pdf中划句子ai伴读的功能，修复pdf高亮显示的问题

addon/locale/en-US/addon.ftl

@@ -101,6 +101,16 @@ lecture-sentence-list-title = Sentences (original, then explanation)
 lecture-sentence-label = P{ $p }·S{ $s }
 lecture-sentence-list-hint = Sentences in this paragraph
 lecture-pdf-sync-hint = The matching passage is highlighted in the PDF reader while playing.
+lecture-explain-selection = Explain & read selection
+lecture-explain-selection-short = Explain
+lecture-selection-empty = Select a sentence in the PDF first
+lecture-selection-explaining = Generating explanation for selection…
+lecture-selection-done = Selection explanation ready
+lecture-selection-no-item = Cannot resolve the current PDF item
+lecture-selection-title = Selection explain
+lecture-selection-syntax = Syntax / terms
+lecture-selection-context = Context role
+lecture-selection-replay = Replay explanation
 lecture-playing-en = Playing: original
 lecture-playing-zh = Playing: translation
 lecture-status-ready-step2 = Ready: { $count } paragraphs + note synced