When generating a document in Templafy the spell check is automatically set to the language chosen in the user profile. Additionally, all text in the document is translated to the chosen language as well.
When generating a presentation in Templafy, only the placeholders on the slides of the initially download is changed to the language chosen in the user profile.
PowerPoint's placeholder text ("Click to add title") follows the language of the preference set in Office Language Preferences. Additionally, the spell check in PowerPoint follows the same preference, but will remember the last setting you changed it to be.