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The official guidance from UK Visas and Immigration for Standard Visitor Visas states:
If you submit a document that is not in English or Welsh, it must be accompanied by a full translation that can be independently verified by the Home Office.
Each translated document must contain:
confirmation from the translator that it is an accurate translation of the original document
the date of the translation
the translator's full name and signature
the translator's contact details
That document goes into more detail on the types of documents that are looking for (and the types of documents they don't want to see; if you send these, they'll know you haven't read the guidance). They specifically suggest a letter from your employer, on letterhead, "detailing your role, salary and length of employment." So your idea to provide that would seem to be in line with what they're asking for, instead of having your entire employment contract translated.
As noted in the comments, your bank and other entities may be able to supply documents in English, which would reduce or eliminate the need for translation.
In addition to the guidance linked above, the 'official' rules are at Appendix V