Localize "by" and "in" strings in post meta

This became a problem when I added right-to-left language support.
When you mix RTL languages like Arabic with English (even for just
dates and small strings) everything becomes jumbled. For now I have
only added translations for English (default), Arabic, and Bulgarian.

i18n/ar.yaml

@@ -36,3 +36,12 @@
 
 - id: links
   translation: "الرَِّوَابِط"
+
+- id: authoredBy
+  translation: "الكاتب"
+
+- id: postedIn
+  translation: "في"
+
+- id: comma
+  translation: "،"

i18n/bg.yaml

@@ -36,3 +36,9 @@
 
 - id: links
   translation: "Линкове"
+
+- id: authoredBy
+  translation: "от"
+
+- id: postedIn
+  translation: "в"

i18n/en.yaml

@@ -36,3 +36,12 @@
 
 - id: links
   translation: "Links"
+
+- id: authoredBy
+  translation: "by"
+
+- id: postedIn
+  translation: "in"
+
+- id: comma
+  translation: ","