Starting with version 3.01.0, Logtalk is distributed under the Apache License 2.0. The license change was kindly sponsored by Kyndi Inc.
The Apache License is a permissive license that requires attribution but allows derivative works to use their own licensing terms (see the text of the license for details). However, as Logtalk requires a backend Prolog compiler to run, the final license of a Logtalk application may depend on the combined result of the Apache License 2.0 and the license of the used backend Prolog compiler.
The following web resource may help in determining the final license of a combination of software using different open source licenses: