Their meaning can even change in time. Quine's underdetermination theory originates in the sciences. For every empirical datum there can be an infinite number of theories that explain it. Science simply picks the combination of hypotheses that seems more plausible. When an hypothesis fails, the scientist can always modify the other hypotheses to make it hold.
Language is a special case.
An empirical datum is a discourse and a theory is its meaning. There are infinite interpretations of a discourse depending on the context. The meaning of language is not even in the mind of the speaker. It is a natural phenomenon related to the world of that speaker. A translation depends on the manual of translation that has been chosen.