Exactly

Some devs with stong opinions have forked X11:

https://x11libre.net/#about

One reason is probably that the power in PS is a 36 % larger number than in kW.

Yes, Stärke means strength (could also mean starch, but not in this case), Stärken here is (Dative case) plural.

How are cars advertised in Portugal? In Germany, Pferdestärken (PS) is commonly used.

You’re right, yet there are two definitions of the horse power, the imperial 1 hp ≈ 745.7 W and the metric 1 hp = 735.49875 W. The latter is usually given in its local name, e.g. as PS, cv, … etc., while the imperial is usually hp independent of the language of the document.

As you said, in Europe, quantifying the power of a machine (e.g. a car) in horse powers is supplementary only and thus, does not suffice on its own, the data must always be given in kW.