We are speaking the same language. Yet we express different propositions when we say "I'm sitting at a table." This is not trivial; for example, various other sentences do not have this property. So the SEP quote, once interpreted correctly, is non-trivial, too, because it is quite clear that the writer did not intend your "speaking different languages" interpretation.
Also, you can divorce a technical notion of a sentence from that of a string of sounds; sentencehood might be a two-place predicate of a string and a language.
But I could easily claim that the way strings like "me" and "here" change their meanings depending on context just shows that we do not always speak the same language. I could think of a language as just being a mapping from symbols to propositions, in which case any variation in propositions expressed means that it is not the same language.
You could argue that there is some kind of mapping to an intermediate state we have in common: symbol -> intermediate -> proposition where symbol -> intermediate is your "language"...
Another month has passed and here is a new rationality quotes thread. The usual rules are: