Programming language design and programming language implementation are very different things.
Theoretically, that's true. In practice, though, for most languages (certainly, almost all of the languages that are likely to be asked about on PLD Stack Exchange):
- The language has only one implementation,
- The implementation is developed by the designer(s),
- The implementation process yields insights which inform adjustments to the design.
So, a community of experts on programming language design will also necessarily be a community of experts on programming language implementation; and Q&As about implementation will be of interest to largely the same audience as Q&As about design.
Therefore, my opinion is that questions about implementation should be welcome and considered on-topic.
Questions about implementing existing languages should be considered entirely off-topic.
I disagree. Q&As about implementing existing languages are still useful to those designing and/or implementing new languages. For example, if somebody asks how to implement Java's static checks for definite assignment or reachability, the knowledge contained in the answers will be relevant to anyone implementing the same thing for a different language.
A rule forbidding implementation questions for existing languages would essentially be a requirement that such questions be phrased in a language-neutral way. The same questions would still be allowed, but the person asking them wouldn't be allowed to give context that would help others to understand the question. I don't see any advantage of such a rule.