The point of view of a dilettante (me, in other words :-))

The problem about any legislation, including copyright legislation, is that it differs (dramatically sometimes) from country to country. General principles of copyright laws in sphere of literary and artistic works are outlined in Berne Convention for the Protection of Literary and Artistic Works that sets the copyright expiration date, as far as I know, to at least 50 years since the author's death or since the first publication of the work if it happened after the author's death.
But, naturally, the copyright law is a very, very attractive way to make money directly from the air, so many countries extend this period. In Russia, for example, we have 70 years term and other countries may set even longer terms.
Another problem is that the expiration date may depend on particular publication date to meet the letter of the law that was active at that time. This may cause funny effects when the older works are still under the copyright protection while the newer ones are already in public domain.
The all mentioned above considers full works, but, apparently, Tatoeba uses only quotations. Copyright laws may allow this type of usage. For example, Russian laws permit free quotation of a copyrighted work without explicit notification/agreement of the legal owner in informational, scientific, educational and cultural purposes provided that the author name and the publication sources are mentioned. I suspect, other countries may have similar principles in their legislation.

My conclusion is the following: Tatoeba is an international project so it should meet all possible copyright laws to avoid being banned in some country for copyright violation. This means we either have to make a research before posting sentence from any potentially copyrighted sources, or to just avoid posting anything newer than, say, 150 years.