Strange as it may seem, there is no standard for the use of the non-SI units used in the USA (as far as I know!).
Traditionally, the symbols are considered to be abbreviations of English (or Latin) words and so are written with a dot after them, and often have the letter s to indicate the plural. Some of them special symbols as well. The following are all equivalent:
5 feet, 5 ft; 5 ft. 5′
2.5 inches; 2.5 in; 2.5 ins: 2.5 in.; 2.5 ins.; 2.5″
10 pounds; 10 lb; 10 lbs; 10 lb.; 10 lbs. 10# [lb is from Latin libra]
The first issue for a translator is whether these terms are words to be translated or symbols to be left in their original form. All major languages have words for “inch, foot”, etc., and some translations use them. (Foot is a body part, and “inch” is often equivalent to “thumb” (French pouce), or “thumbsize” (Spanish pulgada)).
In the end it’s the customer who needs to decide, but at QuickSilver our standard recommendation is:
– in continuous prose, where whole words are used, translate them, e.g. “two inches” = “zwei Zoll” (German), 3 feet = “tre piedi” (Italian”).
– where symbols or abbreviations are used, leave them alone, e.g. 2″, 3 ft, but follow ISO formatting rules.
– ISO rules mean you should omit points and plurals, so 2.54 lb (not 2.54 lbs or lb.)
The last of these ensures a consistent look in translations where the numbers need to be displayed together with metric (SI) equivalents:
2,54 lb (1,55 kg) or 1,55 kg (2,54 lb)
Note the decimal comma and obligatory space before the unit symbol. The essential implication of the above is that what started out in English as abbreviations (with dots, translatable) have now language-independent symbols.