Language acts simply as a constituency signal (“not in my language? Not a voter of mine...”). That is expected and even legitimate - why should a MEP listen to other voices over the ones of people they actually represent? If you really want to influence a MEP, you would get better chances by finding allies in his constituency. This is not really different than with national parliaments.

In terms of this or that national block outvoting a position, it is less frequent than one would expect, and depends largely on how European parties organise. Most parties have some sort of nominated board that agrees a line for the entire group, regardless of national boundaries. Some countries might have a larger influence on the group because of their electoral dynamics (German MEPs for the Greens, for example, will outnumber Italian ones to ridiculous degrees; and some parties are single-country, typically the isolationist ones), but that’s usually not the case in major parties (PSE and PPE).