Does Shakespeare use any figurative language in Sonnet 138?
Shakespeare uses synecdoche in line 7 of Sonnet 138, when he writes "simply I credit her false-speaking tongue. Synecdoche is a figure of speech in which the writer uses a part of something to indicate the whole; in this case, "tongue" indicates "mouth." The question is, why does Shakespeare call attention to her tongue when he could simply say she, or her mouth, spoke lies? In a playful way, Shakespeare draws the reader's attention to the sensual, physical nature of the speaker's relationship with the woman in question. It is also somewhat more coarse or vulgar to invoke the image of a tongue than simply saying that she lies. It contributes to the tone of intimate bickering and base behavior the sonnet describes.
The sonnet also features personification in line 11: "oh, love’s best habit is in seeming trust." Personification is the process of endowing a non-human entity or idea with human qualities. Essentially, the speaker of the poem says that love behaves best when there is trust. This means that love between the two people in the poem is at its best when the two people in question trust one another.
So why write that it is "love's best habit," when really it is the behavior of two human beings? Shakespeare chooses to personify love to show that the particular relationship of the two subjects in the poem applies to a much wider range of people and situations. Not only the two subjects in the poem, but people in all different configurations of romance or friendship display their best loving qualities when they trust in one another—even if the trust is not based entirely on the absolute truth. That is Shakespeare's main point here, that the hard-and-fast truth is not nearly as important as the trust and compatibility between two people if love is the goal.
As the first commenter suggested, there isn't as much figurative language in this sonnet as might be in some others. However, in addition to the pun he or she identified in the word "lie" in line 13, the word "habit" in line 12 is another example of a pun. Here, "love's best habit" can refer to both a habit as a pattern of behavior as well as a habit as something that someone would wear, conveying the idea that love is best clothed, so to speak, in "seeming trust" (line 12). This denotation could be interpreted as comical as a result of the narrator's complicity with the idea of being dishonest in love. However, both denotations are equally appropriate in this context.
Additionally, you could make the case that line 1, "When my love swears that she is made of truth," the lover (the speaker's mistress) is employing hyperbole, or exaggeration, in order to convince the speaker of her veracity. She isn't actually "made" or composed of truth, as though truth were a tangible substance; she is merely insisting that she is honest.
Sonnet 138 is not exactly filled with traditionally poetic devices such as metaphors or imagery. It is a more direct statement of the situation between the speaker and his lady. The central point of the sonnet is that they both lie to each other: he lies that be believes her to be a chaste maid, and she lies that she believes him to be a young man. It is a kind of mutual deception that connects them to each other. The one clever use of language is the pun on the word "lie." In line 13 the speaker says "I lie with her, and she with me," but this can be taken to means that they both tell lies to each other and that they lie together in a physical sexual way as lovers. They are happy together because they both accept the other as they actually are. This poem comes late in the sequence and is likely to be talking about the "Dark Lady" -- a lover that is referenced in several of the sonnets.