I haven't been able to find an obvious translation for to maintain a skill. There are three verbs I've come across that seem like they could do that job:
But I can't seem to figure out which one would be most appropriate, or in which situations one or another may be more appropriate.
Among the verbs you mentioned I would see erhalten as best fitting. But I don’t recommend a word-by-word translation in this case. I think you would need to find a phrase with similar meaning. This could depend on the context. One that could work in many cases is
in Übung bleiben
Your example
I’ve learned to speak German, and must maintain that skill.
could be translated this way
Ich habe Deutsch gelernt und muss in Übung bleiben.
But an alternative for this example would also be
Ich habe Deutsch gelernt und will es nicht wieder verlernen.
This also makes clear that you want to practice to maintain (and not loose) your skill. Nicht verlernen would probably sound strange with skills you cannot possibly learn and/or loose. (But maybe there wouldn’t be much need to maintain them either?)
Two options that I would consider are:
Fähigkeiten erhalten
Wir wollen kontrollieren, ob die Arbeiter ihre technischen Fähigkeiten auf Dauer erhalten können.
This is more or less the go-to translation for maintaining skills in the professional realm.
A bit more florid approach would be to use the verb pflegen
Humor ist eine Fähigkeit, die man pflegen muss.
Pflegen implies a somewhat more intimate involvement with the maintenence of a skill, and is also the verb used for looking after both living and non-living things.
For future reference:
In situations where appropriate phrasing is difficult to ascertain based on circumstance, I highly recommend searching the phrase in question at Linguee.de. There you will find excerpts from (mostly professional) texts that show the language used in context, which can prove much more helpful than a simple dictionary entry. Hope this can be of use in the future!
I’ll try to provide a literal translation.
Although Fähigkeit can be translated as skill, it’s rather an ability.
Fertigkeit is a skill.
halten
is used for solid objects or metaphorically for beloved ones.
erhalten
could be misinterpreted as to receive, especially when used together with Fähigkeit.
aufrechterhalten
is unambiguous when used with Fertigkeit.
wahren
plays in the same league (it’s one of its synonyms) while shorter, therefore
eine Fertigkeit wahren
For “emotional skills”, like humor (mentioned in the comments), use Wesensart instead. Additionally I think Matthias’ in Übung bleiben" is a very good choice. It covers all grounds and is more colloquial.
As commented above I would translate skill to Fertigkeit. There is a little problem however, since in German you would tend to avoid the abstract skill and replace it by whatever skill is intended. E.g.
I learned data base programming but didn’t maintain that skill
would translate to
Ich habe Datenbank-Programmierung gelernt, aber die Tätigkeit/Fertigkeit nie ausgeübt.
This sounds only a bit strange, but in case of bicycling, swimming, French conversation you would use that directly instead of referring to a skill and appearing arrogant.
Ein Skill ist eine Fähigkeit, die man nicht von Geburt an hat. Man muss sie erst erwerben und/oder durch Übung erlernen.
Eine solche Fähigkeit muss man üben (to train), damit man sie nicht verliert. Der Anglizismus trainieren (to train + Endung -ieren) hat sich aber eingebürgert und ist im deutschen Sprachgebrauch üblich.
