The haftara read on the first Shabbat following Shiv'a Asar Be-Tamuz (the first Shabbat of the "three weeks") is taken from the beginning of Sefer Yirmiyahu, and records that prophet's inaugural prophecy. We read that upon hearing God speak to him the first time and assign him the role of prophet, Yirmiyahu responds, "Ah, Lord God! I don't know how to speak, for I am still a boy" (Yirmiyahu 1:6). Rashi and the Radak explain that Yirmiyahu saw himself incapable of administering tokhecha (words of reproach) because nobody would accept the reproach of a youngster. His youth rendered him unsuitable for the position of authority and respect necessary to gain an attentive ear on the part of his audience.
Metzudot David, however, explains Yirmiyahu's concern much differently, as referring to his lack of linguistic skill. He felt incapable of formulating the content of his prophecies eloquently, and hence deemed himself unqualified for the role of prophet.
Abarbanel, in the introduction to his commentary to Sefer Yirmiyahu, also adopts this position, and on this basis arrives at a somewhat startling conclusion. While lauding Yirmiyahu's prophetic stature, noting that Chazal spoke of him more highly than of most other prophets, his skills of literary expression left what to be desired. Abarbanel observes that the poetic quality of Sefer Yirmiyahu pales in comparison with that of other prophets. Furthermore, Yirmiyahu has a tendency to confuse the Hebrew words el (too) and al (above, or about) and to employ incorrect conjugation of verbs. For example, in many instances in Sefer Yirmiyahu, the plural form appears where the singular form is appropriate (and vice-versa), and verbs are often conjugated in the feminine form when modifying a masculine noun (and vice-versa). In addition, Abarbanel observes the inordinate number of instances in this book where words are spelled differently than they are pronounced (keri and ketiv). He attributes these literary deficiencies to Yirmiyahu's youth, claiming that despite the clarity and irrefutably lofty stature of Yirmiyahu's prophecy, he was inexperienced in the art of literary expression, as manifest in the relatively poor linguistic quality of his sefer.
Malbim, in his introduction to Sefer Yirmiyahu, cites and strongly disapproves of Abarbanel's conclusions. One is simply not entitled, Malbim contends, to cast judgment on the literary quality of a text transmitted through prophecy. Malbim here expresses his view that the precise formulation of prophecies, as recorded in the Sifrei Nevi'im, was transmitted to the prophet. God conveyed to the prophet not only the basic content, which the prophet would then formulate in his own words and transmit to the people, but also the precise wording used. Thus, one has no right to attribute a prophet's literary style to his own particular skill or lack thereof.
Abarbanel, of course, works off the assumption that prophets indeed formulated their prophecies in their own, personal style, and thus linguistic tendencies most certainly affected the text used in conveying their prophecies.
Though admittedly I have not conducted a thorough survey on the subject, the more common approach appears to be that of Abarbanel, that all prophets besides Moshe chose their own formulation. In fact, this is one of the common explanations of the Midrashic passage cited by Rashi in his comments to the opening verse of Parashat Matot. The Midrash notes Moshe's introductory remark, "Zeh ha-davar asher tziva Hashem" – "this is the matter that the Lord has commanded." Other prophets, the Midrash observes, begin their prophecies by saying, "Ko amar Hashem" – "Thus said the Lord"; only Moshe could say, "Zeh ha-davar." This distinction is commonly understood to mean that only Moshe reported God's message to the people word-for-word. "Zeh" connotes carbon-copy precision, a quality that characterized only Moshe's prophecies. Other prophets conveyed God's word in their own style; while the content, of course, was authentic, the literally expression used in transmission was the prophet's own creation. But Malbim, as we saw, disagrees, claiming that all prophets conveyed word-for-word reports of the message they had received from the Almighty.