It should just be a case of localizing the Dialogue Waves, that’s what our Audio Localization tools do at least (a localized Dialogue Wave, referencing culture-specific Sound Waves).
We do have some tools to help with Audio Localization, but they’re really designed that you use them from the get-go (they handle things like creating Dialogue Sheets for recording, and batch iterative importing of recorded data and dialogue amendments).
Since you already have your dialogue, you’ll just have to do that part by hand, but it sounds like what you’re doing is already correct as it’s just a case of creating localized assets. So if your source Dialogue Wave was “/Game/Dialogue/HelloWorld” and you wanted to localize it for German, you’d create “/Game/L10N/de/Dialogue/HelloWorld” (which the Asset Localization menu will do for you), and point it to reference the German Sound Waves (which may also be under L10N/de, but don’t have to be overrides of specific assets).
How are you testing this? Cooked game, editor in -game, or PIE? (asset localization doesn’t work with PIE).