You just wrote too many topic sentences into this paragraph. A thesis can be multiple sentences. Usually, it's just a topic sentence that sums up the entire essay.
Look at it this way:
Topic: A statement about the fact(s) with an additional conjecture (often a feeling or another in-concrete expression) expressing your interpretation of the facts that are presented throughout the paragraph.
Body/meat: A fact(s).
Explanation: A statement about the fact(s). (People often interchange explanations and body. That's fine.)
{Repeat body and explanation}
Summary: Reinforces the topic sentence and often flows into the following paragraph.
Other sentence types to make note of are transfers, hooks, and setups for facts. These are not to be used as theses, but they are great tools for writing an essay.
My assessment:
By chopping things up with excessive use of conjunctions and interjections such as "but", "however", "therefore", "and so", and also littering unnecessary punctuation throughout the article, I feel that you have made things overly difficult to follow and committed the fallacy of "complicating the issue". You can find your favorite sum-up topic sentence and select it for the thesis.
What I am seeing, instead of an essay, is either a huge list of attempts at thesis statements, or a confused rant that does not flow organically-consistently from thought-to-thought. And the fact is, the essay isn't just over-complicated with unnecessarily complex grammatical structures. It also uses improper grammar. For example, the colon in the first sentence-type thing. So, rather than trying to be super deep and abstract as often as possible, you should save the deep stuff for the topics, the really deep and abstract stuff for the thesis, and otherwise add more facts, everywhere.
Tagging specific sentences/items in the composition:
Now, it is clear that the decline of a language must ultimately have a political and economic antecedent. (Transfers from a different paragraph. Good topic but not a thesis because of the word "now". I changed "cause" to "antecedent" because you did not set up causation or even correlation with factual statements, anywhere in your essay. An antecedent is something that entices but does not necessarily cause change.) It is not due simply to the bad influence of this or that individual writer. (It should be replaced with something more concrete. If "it" is referring to "the decline of a language," what you are saying seems blindingly obvious. Because of that, you can treat this sentence as a "fact". However, please note that this fact is not organic to the paragraph. The only sentence that proceeds it, the topic sentence, does not set up a need to defend individual writers. This statement belongs in a paragraph that begins with a possible accusation from an external source, targeting individual writers.)
An effect can become a cause, reinforcing the original cause and producing the same effect in an intensified form, and so-on, indefinitely. (This is a little wordy. It pretty much belongs in a philosophy paper. "An effect can become a cause." Is this a definition? "Reinforcing the original cause and producing the same effect in an intensified form." Now, the definition is becoming extremely complex. You can just say, "I will use the words "effect" and "cause" interchangeably in this essay, during an intro, or you can find vocabulary specific to what you are trying to describe. For example: agent, stimulant, agitator....)
A man may take to drink because he feels himself to be a failure, and then fail all the more completely because he drinks. (This sounds like a proverb. Again, very distant from the initial discussion topic, it seems. Readers need to constantly be reminded of what they are reading about. Proverbs are usually used in summaries. Sometimes they are also used in topics under the guise of a 'hook'. I personally don't like that.) It is rather the same thing that is happening to the English language. (Now, we are getting back to English's being effected, somehow. Try not to use the word "it" so hap-hazardously. Referring to a prepositional phrase with a pronoun is a little complex and difficult for even an advanced reader to hold in memory. Also, we used to call pronouns "dead words", in grade school.)
It becomes ugly and inaccurate because our thoughts are foolish, but the slovenliness of our language makes it easier for us to have foolish thoughts. (Again, reaching with "it". This time, reaching over two sentences, perhaps? You have to draw the line, somewhere. Anyway, pending the replacement of your pronoun, this has the potential to pan out into a nice topic sentence or thesis statement. Combine it with the proverb, and you may have a winner?)
The point is that the process is reversible. ("The process" is a substitution for a prepositional phrase that has been replaced by a pronoun. I'm not going to reiterate what I have already said three times, though. This is a "statement about the facts" or "example". This should come after a fact or, for lack of a better word, a quote. The quote should say something to the effect of: "The human process of resisting the reality of failure and the consequences of failure--by engaging in stereotypical self-destructive behaviors--is reversible".)
Modern English, especially written English, is full of bad habits which spread by imitation and which can be avoided if one is willing to take the necessary trouble. (This is a good thesis statement; however, I would argue that Modern English is full of pidgins and dialects which are most typically classified and interchanged from one situation to another, rather than denounced, by professional linguists.) <--THESIS-->
If one gets rid of these habits one can think more clearly, and to think clearly is a necessary first step toward political regeneration. (Another thesis statement along the same lines as the first. You could use this for a summary paragraph or summary sentence.) <-- SUMMARY/RESTATEMENT OF THE THESIS -->
So, the fight against bad English is not frivolous and is not the exclusive concern of professional writers. (Statement regarding the facts/statement about the facts. Needs facts. Remove "so", please.)
I will come back to this presently, and I hope that by that time the meaning of what I have said here will have become clearer. (Transfer. Good for a topic sentence but not a thesis.) Meanwhile, here are five specimens of the English language as it is now habitually written: (Followup to the topic and setup for facts. This is acceptable in the same paragraph as the transfer. Use a colon and create a list. Readers love lists. They'll skip everything else and just read the list, sometimes.)
I recommend reorganizing your thoughts. Organization will keep you from spending hours trying to figure out why your essay doesn't flow. Re-read what you have written. Try a basic outline. Books on the GRE will give you some great ideas to use when approaching a variety of topics.
Also, before you write an essay, gather quotes and citation data. If you are writing a 5 page essay, I recommend finding 2.5 pages worth of quotation data, to use as facts relevant to your hypothesis/hypotheses regarding the topic. Then, organize the quotes to flow with each other, naturally, alongside your interpretation and possible opinion on the question in debate and other evidence.
Finally, may I recommend a brilliant program, which I have fallen in love with? It's called "Readplease 2003". It does not work on Vista or Linux, unfortunately. The duchebag programmers won't release a Linux version, no matter how much I bribe and beg. >X> If you are on XP, though.... I won't change OS's because I love it so much.