When Should You Use Foreign Pronunciations of Foreign Words Outside Their Original Language?

Ahmed, a native speaker of Arabic who also speaks French, wonders how he should pronounce words from those languages when speaking English. For example, should he model the pronunciation for the name of the writer Kahlil Ghibran the country of Qatar on Arabic, or the pronunciation of croissant on French? Or is it better to use the anglicized version? This is part of a complete episode.