When a site links to another it passes that site some of its authority and SEO benefit. Only links that are 'follow' will do this, a nofollow link will not. These values are set by the site owner, often on blogs etc all links are set to nofollow to avoid comment spam where people are trying to get links to their site.

If your comments and links are relevant and add value to an article by all means comment and drop a link, you will be able to check if they are no follow or not to. This post by Rand, although not written to answer this specific question, is useful to illustrate the difference between these links and their relationship in passing link juice.

A higher domain authority site as a rule of thumb is generally better yes, but there are other important factors involved that Google will take into account, then there are considerations about having a link in the comments section as opposed to elsewhere on a page. Again i recommend a read through this post by Rand, Not all links are created equal.

The two articles above should help familiarise yourself with the nuances of link building and the more advanced issues, but if you would like a simple yes no answer then yes, leaving a link in the comments section of a article that is a follow link, can help your SEO. But use this tactic as one part of a balanced link building strategy.

Is this in response to a notice from Google or general house cleaning? And you need to asses if those links are actually causing you any harm in the first place, for example are they nofollow?

First step in removing links is always to try and contact the webmaster and ask them to remove the links. this is important as this will then impact on all Search engines, not just Google which would be the case if you automatically went for Disavow tool. It also provides you an audit trail if in future you find you need to demonstrate to search engines, say through a reconsideration request, that you have made an effort to clean up your site and comply with webmaster guidelines, To find contact details for a website you can try contact details on the site itself or look them up with tools such as the whois directory.

If contact fails, or if they demand money for removal then you should consider using Googles Disavow tool. Again please note this will only impact Google's algorithm. It should only really be a last resort however. You can access this through Webmaster tools, just search for disavow tool in the help section to the top right and follow the links,

I forgot to include, keywords do not need to be exclusive to an individual page, they can be targeted on multiple pages, but having the specific landing page will likely be the most effective area on the site for concentrated on-page SEO tactics.

In respect to Google and other search engines they will determine the language based on the content. So other than having unique URLs for the different content normal site structure considerations come into play. I would also be inclined to use a subfolder and Google's webmaster guidelines for multi lingual sites show no preference.

Heres the relevant bit:

Google uses the content of the page to determine its language, but the URL itself provides human users with useful clues about the page’s content. For example, the following .ca URLs use fr as a subdomain or subdirectory to clearly indicate French content: http://example.ca/fr/vélo-de-montagne.html http://fr.example.ca/vélo-de-montagne.html

Signaling the language in the URL may also help you to discover issues with multilingual content on your site.

Hope this helps.

I would also agree with Baldea and David here, be careful not to Keyword stuff the Home page.

You can include your desired terms within elements of the page if they are relevant / to help the home page rank for those terms, but a product specific page is always going to give you the most scope to target all page elements effectively.

The Page title is only one element, SEOmoz's on page optimisation tool should be useful for you in working out how effectively you are targeting the phrase and what trade-offs are available to you.

Ok, well in wordress you can access the HTML by going to Appearance>editor and you will see the options on the right hand side. However do you use any SEO plugins such as Yoast?

Ok, well you can set canonical tags in Yoast. Without seeing you site set up or the offending pages i am hesitant to give more detailed advice. Best of luck.

In addition to Lynns point it would be worth discounting these URL parameters in Google webmaster tools and bing in order to avoid any possible duplicate content issues, rather than leaving it to them to work out which will also avoid the case Lynn mentions for organic search (A case we also expoerienced with a client!)