What's Happening in the Brain
Language is a distinctly human brain function. It chiefly involves circuitry on the left side of the brain (in people who are right-handed, and most who are left-handed). Language loss is classically called 'aphasia' or 'dysphasia', and is divided into many types, depending on the pattern of loss. The pattern of loss corresponds, although only roughly, to distinct areas of the brain which are involved in this complex circuitry. People with Alzheimer's disease show a large number of language changes, much of which is demonstrated in formal language testing.
Caregivers usually are concerned less with the results of language tests than with what they hear the person that they care for say, and often too, in how they might express themselves in writing. Considering how people speak, those with Alzheimer's disease tend to 'talk around' their subject, rather than get straight to the point. They increasingly use phrases such as "I don't know" and "you know", and will substitute words and phrases like "that thing" or "the whatchamaycallit" for common nouns. These often exist even though conversation is otherwise fluent, they understand what is being said, and the structure of the sentences remains preserved.
Without treatment, language function changes over time, although not always in ways that we might expect. For example, sometimes people have more word substitution when they are aware of the loss of the right word, and want to fill in a description. Later, they are less aware, and they speak less often, without the need to complete a sentence, or even to 'make sense'.