Evidently, many folks misunderstood Askjo's assertion, by no fault of their own, since he refused to make his argument explicit. It wasn't about the phrases "is not true" and "is true". The two verses without context from the KJV --
If I bear witness of myself, my witness is not true. (John 5:31)
Jesus answered and said unto them, Though I bear record of myself, [yet] my record is true:... (John 8:14a)
What he is claiming is that the KJV escapes the appearance of contradiction because John 5 uses the English word "witness", while in chapter 8 the word "record" is used. This is an utterly ridiculous (desperate) semantic excuse. Any appeal that these two English words are NOT synonymous is tantamount to accusing the 1611 revisors of outright mistranslation; to render the same underlying Greek word (with very similar context) with dissimilar meaning English words would be undeniable error. No, the apparent contradiction still exists in the KJV; ask any Atheist, they'll be glad to show it to you.
The Greek verb
martureo (Strong's #3140) means to bear witness, the action of giving testimony. The verb form then is actually two words in KJV English: "bear witness" and "bear record". The Greek noun
marturia (Strong's #3141) is that which one testifies, the verbal testimony itself (i.e. before a judge). The KJV's less familar rendering of the verb form obscures clarity for contemporary English readers; commonly other versions use a verb form (like "testify") with a corresponding noun form (like "testimony"). For example --
If I {alone} testify about Myself, My testimony is not true. (John 5:31, NASB)
Jesus answered and said to them, "Even if I testify about Myself, My testimony is true,... (John 8:14a, NASB)
The conjuction "if" has several uses depending on the intended meaning and can sometimes create ambiguities: "if" can indicate uncertainty, or "if" can be conditional, or "if" can be used in contrary-to-fact statements (hypothetical).
Regardless of the translation, carefull handling of the Word of God in context will reveal that there is no contradiction at all between these verses.