The dictionary is configurable at the text level so different paragraphs can be spell checked with different dictionaries. To fix this comprehensively, you need to ensure all paragraph styles have your chosen language applied to them and also that there is no local language settings applied that would override the style language setting. This should be done in your template first and then applied to each existing document.
There are hundreds of styles in even the most basic template so this can be a big task that is simplified greatly by using a macro.
Code:
Sub SetLanguage()
'Changes all the styles in the document to UK spelling
Dim aStyle As Style
For Each aStyle In ActiveDocument.Styles
If aStyle.Type = wdStyleTypeParagraph Then
aStyle.LanguageId = wdEnglishUK
End If
Next aStyle
ActiveDocument.Range.LanguageId = wdEnglishUK
End Sub