I think that in English the distinction normally made is between woodfree (sans trace de bois), used to refer to paper made from chemical pulp and containing little or no mechanical pulp (although up to 10% is allowed, even though it is referred to as "free"), and wood-containing (avec trace de bois), used to refer to paper containing a proportion of mechanical pulp. Your term Légère trace de bois then refers to something in between - reference can be found to almost woodfree, virtually woodfree, mainly woodfree, semi-woodfree, containing some wood, etc.

Here are a number of links.

Avec bois (papier ~) : Papier contenant une certaine proportion de pâte mécanique (de 40 à 70 %).
Wood containing (paper ~): Paper containing a certain proportion of mechanical pulp (from 40 to 70%).
Sans bois (papier ~) : Papier entièrement fabriqué à partir de pâtes chimiques et exempt de toutes impuretés du bois telles que la lignine.
Wood free (paper ~): Paper entirely made from chemical pulp and free from wood based impurities, such as lignin.
http://fr.bollorethinpapers.com/resources/knowledge-training...
http://us.bollorethinpapers.com/resources/knowledge-training...

Pourquoi parle-t-on de papier "sans bois"?
Dans le langage courant "sans bois" signifie "sans trace apparente de bois", c’est à dire une cellulose préparée chimiquement et non mécaniquement.
Le procédé mécanique ne permet pas d’éliminer totalement les "traces de bois" et de lignine.
http://www.inapa.lu/subpage.php?page_ID=6&subpage_ID=27

Woodfree paper
A printing and writing paper which contains little or no mechanical woodpulp. For statistical purposes any paper which contains less than 10% mechanical pulp is categorised as woodfree. If the proportion is greater than 10% it is categorised as mechanical paper. Woodfree paper may be coated (CWF) or uncoated (UWF). Woodfree paper is sometimes known as "fine paper". In the United States woodfree paper is known as "free sheet".
Chemical pulp
A generic term which describes pulp produced by chemical (as opposed to mechanical) processes. These chemical processes include the kraft (or sulphate) and sulphite processes.
Mechanical pulp
A generic term describing pulps produced by a mechanical (as opposed to a chemical) process. Also known as "high-yield" pulp as the processes utilize a higher proportion of the wood raw material than do the chemical processes. The mechanical processes include groundwood, refiner mechanical, thermo mechanical and chemi-thermomechanical. Mechanical pulps are used principally in the production of newsprint and in publication grades of printing and writing paper.
http://www.pulpwatch.com/node/12

Woodfree
Woodfree is a description of pulp and paper meaning that they contain little or no mechanically ground fibres. Implies that fibres are chemically treated, thereby eliminating lignin (the substance that binds wood fibres together in the tree) and making the product purer, whiter and stronger. Woodfree is an historical paper-making term shortened from 'groundwood-free' and does not denote a paper or pulp made from materials other than wood.
http://www.roberthorne.co.uk/contact/glossary/w/

Woodfree paper is mainly produced from chemical pulp fibers and must contain a maximum of 5% ground- wood pulp. For example, a lot of printing and writing paper as well as uncoated paper and finished paper with a coated surface is woodfree paper.
Paper containing wood is produced by using a high proportion of mechanical pulp (groundwood pulp). Paper containing wood still contains a large proportion of lignin (lignified and robust part of the wood) so that it yellows relatively quickly. Examples of paper containing a lot of wood are newsprint and magazine paper which are increasingly produced by using waste paper (secondary fibers/deinked fibers). In general the proportion of mechanical pulp is not always precisely indicated. For this reason the designations "containing some wood" or "almost woodfree" are used.
http://books.google.co.uk/books?id=VrdqBRgSKasC&pg=PA125&lpg...

Wood containing and woodfree paper
Earlier in the section we explained the two principal ways of manufacturing pulp, the chemical method and the mechanical method. The chemical method removes practically all wood-containing substances from the wood, apart from the cellulose.This is why paper that contains over 90 per cent woodfree pulp, i.e. chemical pulp, is called woodfree. A woodfree paper’s properties include high strength and good permanence.
The production of mechanical pulp exploits the majority of the tree volume, which means that parts other than the pure cellulose are added to the pulp mixture. A paper with over 10 per cent wood pulp is therefore called wood-containing paper. The properties of a wood-containing paper include good strength, high opacity and a natural feel.
Paper grades which mainly comprise semi-chemical pulp change the traditional distribution of woodfree and wood-containing paper because benefits are gained from both types of pulp. By definition these are often referred to as wood-containing paper, although they are often more similar to woodfree grades depending on the desired function.
http://www.arcticpaper.com/en/services/Uncoated-guide/Paper/...

A century ago, the divide between wood-containing and woodfree papers was clearly marked. Wood-containing papers, such as news-print, were made primarily with mechanical pulps using spruce stone groundwood, while woodfree papers, such as fine writing papers, were made from rag and sulphite or kraft chemical pulps.
Today, thanks to advances in pulp manufacturing, the distinction between so-called "woodfree" and "wood-containing" papers has blurred significantly.
http://www.allbusiness.com/specialty-businesses/840100-1.htm...