Nintendo UK have noted on their official website that the Japanese audio will not be included in the upcoming Fire Emblem Echoes: Shadows of Valentia. This is notable especially as while Fire Emblem Fates was also dub-only, Fire Emblem: Awakening had the option for Japanese audio included in the game. Here is the full description, via Nintendo UK.
The glorious past is reborn in Fire Emblem Echoes: Shadows of Valentia, only on Nintendo 3DS family systems.
Inspired by Fire Emblem Gaiden, which released exclusively in Japan in 1992, Fire Emblem Echoes: Shadows of Valentia recreates classic Fire Emblem gameplay with a modern twist, mixing in exploration of towns and dungeons.
Dual protagonists Alm and Celica will join others locked in a tragic war dividing the continent of Valentia. Conflicts will come to life in cut scenes created by Studio Khara and every line of dialogue has been fully voiced*.
Free-roaming, dungeons crawling with enemies, flexible character progression – everything that made Fire Emblem Gaiden unique is present in Fire Emblem Echoes: Shadow of Valentia, only on Nintendo 3DS family systems.
*Please note that this game will be available only with English voiceover.
Originally revealed in January of this year in a Nintendo Direct, Nintendo are aiming to release the game this May worldwide exclusively on Nintendo 3DS systems.