Sadly, there are many different possible reasons. It looks like anything other than Windows display language, Windows region, Office display language, Office default editing language, keyboard language and intended editing language all matching has the potential to create problems.
Some possibilities include:
Wrong slide master language. Go to View > Slide Master. Select the parent style, select all text boxes (e.g. with Ctrl+A). Then change the proofing language in Proofing/Review > Language.
In my case, new documents are always in German despite the editing language being set to English. I suspect that the "Language Accessory Pack" for the intended language must be installed to get the intended behavior, which cannot be installed on all systems; The Download page offers only an
.exe installer, which then complains about being incompatible with
.msi based Office installations; On a workplace PC, this may not be fixable. When using a corporate template, it doesn't matter though, since the language apparently needs to be set in the template.
Keyboard language. Some programs may use your systems keyboard language as the proofing language of newly typed text. Note that windows allows configuring any combination of keyboard language and keyboard layout. I for instance use "English (US)" with a German layout.
Editing language. Check the settings under File > Options > Language. For me changing the default language had no effect in Powerpoint though, new slides still start out with the slide master in German, when I want English.
I think this is what you show in your screenshot, so I guess you're making the same experience here as me.