Writing out numbers (especially decimal figures) in words does look cumbersome in English but given this is from a legal document, I don't think it's the translator's job to change things in this way. The words should simply be translated.

Whilst it might not be commonplace nowadays, UK legal draftsmen are known to write out numbers longhand.

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Note added at 1 day 1 hr (2020-01-22 14:09:38 GMT)
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Note that may European civil countries use "thosandths" to divide interests in land. This is partucularly the case with co-ownerhips where the share of the communal parts of a building are allocated in thousandths to the individual owners.

The asker hasn't told us what this particular text concerns, but it may be appropriate here to translate as "three hundred and fifty four one thousandth parts"