Here's the Japanese sales of all the Fire Emblem games going back to the third one, Mystery of the Emblem for the Super Famicom
FE3: Mystery of the Emblem - 697,750
FE4: Genealogy of a Holy War - 578,000
FE5: Thracia 776 - 257,500
FE6: Sword of Seals - 393,000
FE7: Blazing Sword - 293,500
FE8: Sacred Stones - 298,000
FE9: Path of Radiance - 155,750
And even though I don't have the exact numbers, all the Fire Emblems that have come to America have OUTSOLD their Japanese counterparts. That's right, the Fire Emblem series has been more popular here in America over the last several years then in Japan. The American version of FE7 sold around 400,000, I think I saw FE8 around 300,000 and FE9 last time I checked sold over 160,000. Now when you combine the sales of the American and Japanese version of all these games the worldwide numbers would be like this.
FE7: Blazing Sword - 700,000
FE8: Sacred Stones - 600,000
FE9: Path of Radiance - 300,000
So to answer the question of why Nintendo is bothering to translate the series, it's because they're making alot more money with both the English and Japanese version then they would with just leaving the series Japan only.