In the most recent versions of SQL Server, this is actually somewhat simple with builtin functionality. YOu can use STRING_SPLIT to split the value, and ensure you return the ordinal positions so that you can use LEAD or LAG get the next/last delimited value and filter to your EN values. Finally you can use TRIM to remove those trailing commas:
WITH CTE AS(
SELECT V.ARTIKELNR,
V.DESCRIPTION,
SS.value,
LEAD(SS.value) OVER (PARTITION BY V.ARTIKELNR ORDER BY SS.ordinal) AS NextValue
FROM (VALUES(104009400031900,N'S235JRG2C+C EN 10277/10 80 x 8 mm Blanker Flachstahl'),
(104009800002950,N'Werksattest nach EN 10204'),
(105009400092360,N'EN 10060 Inox Rund Ø 28 mm WNr1.4034 warmgefertigt'),
(105009400068571,N'90 mm WNr1.4404 warmgefertigt, EN 10060, geschmiedet'),
(105009400004420,N'WNr1.4301 Inox Vierkantstahl 6 x 6 mm EN 10278, blankgezogen'),
(105009400008800,N'WNr1.4301 Inox Band geschnitten 25 x 4 mm, Breitentol. 25 m'),
(105009400068600,N'WNr1.4112 Inox Rund 8.00 mm nach EN 10088-3 + A+C+SL, gesch'),
(105009400068700,N'WNr1.4301 Inox Rund 36.00 mm nach EN 10088-3, geschliffen, T'),
(105009400068800,N'WNr1.4104 Inox Rund 25.00 mm nach EN 10088-3, geschliffen, T'),
(105009400010035,N'WNr1.4404 Inox Vierkantstahl 30 x 30 mm, gezogen EN 10278'))V(ARTIKELNR,DESCRIPTION)
CROSS APPLY STRING_SPLIT(V.DESCRIPTION,' ',1) SS)
SELECT C.ARTIKELNR,
TRIM(',' FROM CONCAT_WS(' ',C.value,C.NextValue)) AS Norm
FROM CTE C
WHERE C.value = 'EN';
db<>fiddle
In 2012 (reminder, this became completely unsupported in 2022) it's a little harder. You can replace STRING_SPLIT with a different non-builtin splitter that returns the ordinal position, such as DelimitedN4KSplit_LEAD (N4K as your strings appear to be an nvarchar). You still have access to LEAD, but not TRIM, so you'll need to use REPLACE and RTRIM. This gives you something like this:
WITH CTE AS(
SELECT V.ARTIKELNR,
V.DESCRIPTION,
DS.item,
LEAD(DS.Item) OVER (PARTITION BY V.ARTIKELNR ORDER BY DS.ItemNumber) AS nextitem
FROM (VALUES(104009400031900,N'S235JRG2C+C EN 10277/10 80 x 8 mm Blanker Flachstahl'),
(104009800002950,N'Werksattest nach EN 10204'),
(105009400092360,N'EN 10060 Inox Rund Ø 28 mm WNr1.4034 warmgefertigt'),
(105009400068571,N'90 mm WNr1.4404 warmgefertigt, EN 10060, geschmiedet'),
(105009400004420,N'WNr1.4301 Inox Vierkantstahl 6 x 6 mm EN 10278, blankgezogen'),
(105009400008800,N'WNr1.4301 Inox Band geschnitten 25 x 4 mm, Breitentol. 25 m'),
(105009400068600,N'WNr1.4112 Inox Rund 8.00 mm nach EN 10088-3 + A+C+SL, gesch'),
(105009400068700,N'WNr1.4301 Inox Rund 36.00 mm nach EN 10088-3, geschliffen, T'),
(105009400068800,N'WNr1.4104 Inox Rund 25.00 mm nach EN 10088-3, geschliffen, T'),
(105009400010035,N'WNr1.4404 Inox Vierkantstahl 30 x 30 mm, gezogen EN 10278'))V(ARTIKELNR,DESCRIPTION)
CROSS APPLY fn.DelimitedSplitN4K_LEAD(V.DESCRIPTION,' ') DS)
SELECT C.ARTIKELNR,
RTRIM(REPLACE(CONCAT(C.item,' ',C.nextitem),',',' ')) AS Norm
FROM CTE C
WHERE C.item = 'EN';