Is there a way, and if so, how can I create in a short way “Umlaute” (of uUoOaOs) in Kate via keyboard code(s))
TIA
the most accessible way is to use the compose feature.
i can tap the compose key and then type O and " to get Ö
you can set up your compose key location in the keyboard settings under advanced tab
i can tap the compose key and then type O and " to get Ö
That gets U+0150 Ő LATIN CAPITAL LETTER O WITH DOUBLE ACUTE, used in Hungarian Wikipedia tells me, different from U+00D6 Ö LATIN CAPITAL LETTER O WITH DIAERESIS, for which one types compose O :
They look the same on a computer for smallish fonts. I used
ű
for years as u umlaut not knowing the difference.
if you want to see what the actual compose codes are for your language you can look at this file
/usr/share/X11/locale/en_US.UTF-8/Compose for us english
i would type the compose key and then = and O which gives me Ő which is the U0150 unicode
Depending on how often you need this you might also want to have a look at kcharselect, it even has bookmarks for characters you need to use often.
Another option would be to just additionally install a country’s keyboard layout that has the characters you need and switch keyboards using a hot key (but you’ll have to remember the key positions on your physical keyboard)
I tried a few layouts until I decided I wanted stuff that no keyboard got quite right…
O then I just created a custom layout (just after buying a 75% keyboard)…
So the basic layout is now just very plain US English with no 3rd level items, it was blank…
Then I just added what I wanted as suited me, initially just currencies, so on 4, I have $, ฿ and ⁴… e has E,
€ and £, x has × and · for maths, / has ÷…
It’s trivial to pin your custom layout in Kate, so that if there’s anything you need to add, just add another third level spot (that’s a whole new hidden layout).
Then obviously .XCompose means you can add things you won’t remember (I have cream coloured caps, so I can write some on the side of the keys) - memo 
or warning 
it’s magic 
🖉 I also have a second .XCompose which I swap in for phonetics - I had to hash out most of those because when you add too much stuff, it starts to clash; so I can also swap that (with a script called ‘xcom-phonic’).
There’s a lot of 
in there - ⁂ ≈ ≠ ≡ √
Just remember, Kate can also remember sessions - so you could open your .XCompose, as well as the regular Compose (mine’s /usr/share/X11/locale/en_US.UTF-8) to pull it up for reference… it’s not always easy to remember… ①② or ⒶⒷⒸ,
i know, i though i was responding to jlittle
or make your own keyboard layout that has all the characters you use so you can find them in places you expect.
but compose is more easily accessible.
I’ve first ran into this problem in the 1990s when reviewing some Polish music and now I live in Sweden with äöå. I’ve tried different ways through the years (and also on different OSs since work machines often were Windows) like keyboard layouts and hotkeys (and here in Sweden even the Swedish layout with the three extra keys but all the non-letter keys are in different places than I am used to). (For Windows I even wrote a Intl-European definition that had all European characters in Latin script.)
I always came back t the compose option (for which I also found a way to do on Windows). No more key bindings to remember, only the accents.
I sometimes type in Spanish, and set up an alternative keyboard with dead keys. I can switch to it either via the task bar or with a key combo (I use Ctrl-Shift) and can then type diacritics:
áéíóú plus ü and ñ
(I can never remember the Compose key, so I find this easier).